UPDATE: The error mysteriously disappeared.
After upgrading Debian from jessie to stretch, everything is working fine except virtualmin. I get this error:
Can't locate lib.pm in @INC (you may need to install the lib module) (@INC contains: /usr/share/webmin /etc/perl /usr/local/lib/x86_64-linux-gnu/perl/5.20.2 /usr/local/share/perl/5.20.2 /usr/lib/x86_64-linux-gnu/perl5/5.20 /usr/share/perl5 /usr/lib/x86_64-linux-gnu/perl/5.20 /usr/share/perl/5.20 /usr/local/lib/site_perl . ..) at /usr/share/webmin/authentic-theme/authentic-lib.pm line 10. BEGIN failed--compilation aborted at /usr/share/webmin/authentic-theme/authentic-lib.pm line 10. Compilation failed in require at /usr/share/webmin/authentic-theme/index.cgi line 10.``` It seems rather strange that I'd have to install the lib module.