Since the latest update, I have encountered multiple cases of Leapp hitting the default limit of file descriptors per process (You can find them linked to the issue).

The visible error is "OperationalError: unable to open database file". Using strace we can see it fails to open it due to the limit:

70588 16:36:26.909394 open("/var/lib/leapp/leapp.db", O_RDWR|O_CREAT|O_CLOEXEC, 0644) = -1 EMFILE (Too many open files) <0.000023>
70588 16:36:26.909445 open("/var/lib/leapp/leapp.db", O_RDONLY|O_CLOEXEC) = -1 EMFILE (Too many open files) <0.000017>

As a workaround, the limit can be raised temporarily with "ulimit":

# ulimit -n 16384