As per #6563#note-6:

This certificate is both self-signed and outdated (https://www.dropbox.com/s/sx165r59myd8q24/Captura%20de%20pantalla%202018-01-11%2017.17.15.png?dl=0 -- shouldn't Tigase regenerate self-signed outdate certs?!), I've updated the certificate as well.

Yes, the Tigase should automatically regenerate certificate if it is outdated and it used to do it. Not sure why it does not do it anymore.