The qbittorrent package version (3.2.3-2) depends on Qt4 libs so I doubt it has to do with Qt5.
As I see, the installed libc6 package is taken from experimental so I'd not be surprised by temporary failures between upgrades. The bug's reporter did not report back whether the problem went away or not after some subsequent upgrades, nevertheless I didn't observe such a dramatic loss of functionality neither in any stable release nor in the current testing/unstable. I believe this bug can be closed as invalid. Regards BZ
