The problem is in the title: my Foobar2000 installation on a Pixel 3XL stopped working (playing files) after I installed the first public beta of Android 11. The error message notes that whatever media file I select is already in use. Have others experienced this problem?
My guess is that the problem arises from Android's fiddling with file permissions, but I know nil of the guts of nor Andoid nor Foobar2000.
Thanks for reporting.
Apparently Linux flock() function is broken (deliberately removed?) on Android 11. Workaround has been added for v1.2.6 beta.
Version 1.2.6 available in APK downloads (https://foobar2000.org/apk) and soon on Google Play beta.