Slowdown problem solved! :-D
But I managed to provoke the following - oddly enough, only in PowerShell (when it occurred, I tried to give cmd in the same window and the same flac command, and the error did not reproduce). Note the zero percents and the "c" following the one-character-shorter .wav files:
LetUsSeeWhatHappensWhenTheFilenameBecomesAsLongAsThisThisThisLongLongLongLongerLongerEvenLonger11!!!!!eleventyeleven - Copy (2).flac: 0%
wrote 68998568 bytes, ratio=1,000
LetUsSeeWhatHappensWhenTheFilenameBecomesAsLongAsThisThisThisLongLongLongLongerLongerEvenLonger11!!!!!eleventyeleven - Copy (2).wav: 0% c
wrote 68998568 bytes, ratio=0,498
LetUsSeeWhatHappensWhenTheFilenameBecomesAsLongAsThisThisThisLongLongLongLongerLongerEvenLonger11!!!!!eleventyeleven - Copy (3).flac: 0%
wrote 68998568 bytes, ratio=1,000
LetUsSeeWhatHappensWhenTheFilenameBecomesAsLongAsThisThisThisLongLongLongLongerLongerEvenLonger11!!!!!eleventyeleven - Copy (4).flac: 0%
wrote 68998568 bytes, ratio=1,000
LetUsSeeWhatHappensWhenTheFilenameBecomesAsLongAsThisThisThisLongLongLongLongerLongerEvenLonger11!!!!!eleventyeleven - Copy (4).wav: 0% c
wrote 68998568 bytes, ratio=0,498
LetUsSeeWhatHappensWhenTheFilenameBecomesAsLongAsThisThisThisLongLongLongLongerLongerEvenLonger11!!!!!eleventyeleven - Copy (5).flac: 0%
wrote 68998568 bytes, ratio=1,000
LetUsSeeWhatHappensWhenTheFilenameBecomesAsLongAsThisThisThisLongLongLongLongerLongerEvenLonger11!!!!!eleventyeleven - Copy (5).wav: 0% c
wrote 68998568 bytes, ratio=0,498
LetUsSeeWhatHappensWhenTheFilenameBecomesAsLongAsThisThisThisLongLongLongLongerLongerEvenLonger11!!!!!eleventyeleven - Copy (6).flac: 0%
wrote 68998568 bytes, ratio=1,000
LetUsSeeWhatHappensWhenTheFilenameBecomesAsLongAsThisThisThisLongLongLongLongerLongerEvenLonger11!!!!!eleventyeleven - Copy.flac: 0% comp
wrote 68998568 bytes, ratio=1,000
LetUsSeeWhatHappensWhenTheFilenameBecomesAsLongAsThisThisThisLongLongLongLongerLongerEvenLonger11!!!!!eleventyeleven - Copy.wav: 0% compl
wrote 68998568 bytes, ratio=0,498
LetUsSeeWhatHappensWhenTheFilenameBecomesAsLongAsThisThisThisLongLongLongLongerLongerEvenLonger11!!!!!eleventyeleven.flac: 0% complete, rwrote 68998568 bytes, ratio=1,000
LetUsSeeWhatHappensWhenTheFilenameBecomesAsLongAsThisThisThisLongLongLongLongerLongerEvenLonger11!!!!!eleventyeleven.wav: 0% complete, rawrote 68998568 bytes, ratio=0,498
LetUsSeeWhatHappensWhenTheFilenameBecomesFullOfDingbats➀ ➁ ➂ ➃ ➄ ➅ ➆ ➇ ➈ ➉ ➊ ➋ ➌ ➍ ➎ ➏AndAsLongAsThisThisThisLongLongLongLongerLongerEvenLonger11!!!!!eleventyeleven - Copy (6).flac: wrote 68998568 bytes, ratio=1,000
LetUsSeeWhatHappensWhenTheFilenameBecomesFullOfDingbats➀ ➁ ➂ ➃ ➄ ➅ ➆ ➇ ➈ ➉ ➊ ➋ ➌ ➍ ➎ ➏AndAsLongAsThisThisThisLongLongLongLongerLongerEvenLonger11!!!!!eleventyeleven - Copy (6).wav: wrote 68998568 bytes, ratio=0,498
Resizing the window slightly, I think the problem is a combination of 0% (or maybe single-digit percent?!?) and linebreaks:
LetUsSeeWhatHappensWhenTheFilenameBecomesAsLongAsThisThisThisLongLongLongLongerLongerEvenLonger11!!!!!eleventyeleven - Copy (2).flac: 0% complete, ratio
wrote 68998568 bytes, ratio=1,000
LetUsSeeWhatHappensWhenTheFilenameBecomesAsLongAsThisThisThisLongLongLongLongerLongerEvenLonger11!!!!!eleventyeleven - Copy (2).wav: 0% complete, ratio=
wrote 68998568 bytes, ratio=0,498
LetUsSeeWhatHappensWhenTheFilenameBecomesAsLongAsThisThisThisLongLongLongLongerLongerEvenLonger11!!!!!eleventyeleven - Copy (3).flac: 0% complete, ratio
wrote 68998568 bytes, ratio=1,000
LetUsSeeWhatHappensWhenTheFilenameBecomesAsLongAsThisThisThisLongLongLongLongerLongerEvenLonger11!!!!!eleventyeleven - Copy (4).flac: 0% complete, ratio
wrote 68998568 bytes, ratio=1,000
LetUsSeeWhatHappensWhenTheFilenameBecomesAsLongAsThisThisThisLongLongLongLongerLongerEvenLonger11!!!!!eleventyeleven - Copy (4).wav: 0% complete, ratio=
wrote 68998568 bytes, ratio=0,498
LetUsSeeWhatHappensWhenTheFilenameBecomesAsLongAsThisThisThisLongLongLongLongerLongerEvenLonger11!!!!!eleventyeleven - Copy (5).flac: 0% complete, ratio
wrote 68998568 bytes, ratio=1,000
LetUsSeeWhatHappensWhenTheFilenameBecomesAsLongAsThisThisThisLongLongLongLongerLongerEvenLonger11!!!!!eleventyeleven - Copy (5).wav: 0% complete, ratio=
wrote 68998568 bytes, ratio=0,498
LetUsSeeWhatHappensWhenTheFilenameBecomesAsLongAsThisThisThisLongLongLongLongerLongerEvenLonger11!!!!!eleventyeleven - Copy (6).flac: 0% complete, ratio
wrote 68998568 bytes, ratio=1,000
LetUsSeeWhatHappensWhenTheFilenameBecomesAsLongAsThisThisThisLongLongLongLongerLongerEvenLonger11!!!!!eleventyeleven - Copy.flac: 0% complete, ratio=4,4
wrote 68998568 bytes, ratio=1,000
LetUsSeeWhatHappensWhenTheFilenameBecomesAsLongAsThisThisThisLongLongLongLongerLongerEvenLonger11!!!!!eleventyeleven - Copy.wav: 0% complete, ratio=2,22
wrote 68998568 bytes, ratio=0,498
... another minor resize and I get it in a different track:
Anal Trump {2016} That Makes Me Smart! - 06 - Poor People Are Too Stupid To Get A Loan From Their Parents - Copy (10).flac: 0% comwrote 4015735 bytes, ratio=1,000
Anal Trump {2016} That Makes Me Smart! - 06 - Poor People Are Too Stupid To Get A Loan From Their Parents - Copy (11).flac: 0% comwrote 4015735 bytes, ratio=1,000
Anal Trump {2016} That Makes Me Smart! - 06 - Poor People Are Too Stupid To Get A Loan From Their Parents - Copy (12).flac: 0% comwrote 4015735 bytes, ratio=1,000
Anal Trump {2016} That Makes Me Smart! - 06 - Poor People Are Too Stupid To Get A Loan From Their Parents - Copy (13).flac: 0% comwrote 4015735 bytes, ratio=1,000
Anal Trump {2016} That Makes Me Smart! - 06 - Poor People Are Too Stupid To Get A Loan From Their Parents - Copy (14).flac: 0% comwrote 4015735 bytes, ratio=1,000
Anal Trump {2016} That Makes Me Smart! - 06 - Poor People Are Too Stupid To Get A Loan From Their Parents - Copy (15).flac: 0% comwrote 4015735 bytes, ratio=1,000
Anal Trump {2016} That Makes Me Smart! - 06 - Poor People Are Too Stupid To Get A Loan From Their Parents - Copy (16).flac: 0% comwrote 4015735 bytes, ratio=1,000
Anal Trump {2016} That Makes Me Smart! - 06 - Poor People Are Too Stupid To Get A Loan From Their Parents - Copy (17).flac: 0% comwrote 4015735 bytes, ratio=1,000
Anal Trump {2016} That Makes Me Smart! - 06 - Poor People Are Too Stupid To Get A Loan From Their Parents - Copy (18).flac: 0% comwrote 4015735 bytes, ratio=1,000
Anal Trump {2016} That Makes Me Smart! - 06 - Poor People Are Too Stupid To Get A Loan From Their Parents - Copy (19).flac: 0% comwrote 4015735 bytes, ratio=1,000
Anal Trump {2016} That Makes Me Smart! - 06 - Poor People Are Too Stupid To Get A Loan From Their Parents - Copy (2).flac: 0% compwrote 4015735 bytes, ratio=1,000
Anal Trump {2016} That Makes Me Smart! - 06 - Poor People Are Too Stupid To Get A Loan From Their Parents - Copy (20).flac: 0% comwrote 4015735 bytes, ratio=1,000
Anal Trump {2016} That Makes Me Smart! - 06 - Poor People Are Too Stupid To Get A Loan From Their Parents - Copy (21).flac: 0% comwrote 4015735 bytes, ratio=1,000
Anal Trump {2016} That Makes Me Smart! - 06 - Poor People Are Too Stupid To Get A Loan From Their Parents - Copy (3).flac: 0% compwrote 4015735 bytes, ratio=1,000
Anal Trump {2016} That Makes Me Smart! - 06 - Poor People Are Too Stupid To Get A Loan From Their Parents - Copy (4).flac: 0% compwrote 4015735 bytes, ratio=1,000
Anal Trump {2016} That Makes Me Smart! - 06 - Poor People Are Too Stupid To Get A Loan From Their Parents - Copy (5).flac: 0% compwrote 4015735 bytes, ratio=1,000
Anal Trump {2016} That Makes Me Smart! - 06 - Poor People Are Too Stupid To Get A Loan From Their Parents - Copy (6).flac: 0% compwrote 4015735 bytes, ratio=1,000
Anal Trump {2016} That Makes Me Smart! - 06 - Poor People Are Too Stupid To Get A Loan From Their Parents - Copy (7).flac: 0% compwrote 4015735 bytes, ratio=1,000
Anal Trump {2016} That Makes Me Smart! - 06 - Poor People Are Too Stupid To Get A Loan From Their Parents - Copy (8).flac: 0% compwrote 4015735 bytes, ratio=1,000
Anal Trump {2016} That Makes Me Smart! - 06 - Poor People Are Too Stupid To Get A Loan From Their Parents - Copy (9).flac: 0% compwrote 4015735 bytes, ratio=1,000
Anal Trump {2016} That Makes Me Smart! - 06 - Poor People Are Too Stupid To Get A Loan From Their Parents - Copy.flac: 0% completewrote 4015735 bytes, ratio=1,000
Anal Trump {2016} That Makes Me Smart! - 06 - Poor People Are Too Stupid To Get A Loan From Their Parents.flac: 0% complete, ratiowrote 4015735 bytes, ratio=1,000