Comment 3 for bug 1752082

The reason for this happening is that lua-posix changed its main module name from posix_c to posix. The maintainers of the debian package missed that and still install a symlink from /usr/lib/x86_64-linux-gnu/lua/5.1/posix_c.so.

This can be seen in the output above (no file '/usr/lib/x86_64-linux-gnu/lua/5.1/posix.so') which shows that a posix.so is searched for.

The solution for the users is to create a symlink to that file: sudo ln -s /usr/lib/x86_64-linux-gnu/lua/5.1/posix_c.so /usr/lib/x86_64-linux-gnu/lua/5.1/posix.so
Or simply rename the posix_c.so to posix.so
WARNING: This has to be undone before upgrading after a fix from the package maintainers gets released or the upgrade might fail.

The maintainers should fix that in the packages ASAP.