Checked my own logs, and I have good news and bad news.
Good news: On my server user-manager works perfectly fine and does what it is supposed to. I can create procmail rules, check the log, write vacation messages etc. *but* I get the same error messages as you do! E.g. clicking "Process mail" writes these lines in the httpd/admin_error_log:
[Wed Oct 15 19:27:24 2003] userpanel-procmail: Use of uninitialized value in numeric lt (<) at /usr/lib/perl5/site_perl/5.6.0/i386-linux/Unicode/String.pm line 242.
[Wed Oct 15 19:27:24 2003] userpanel-procmail: Use of uninitialized value in numeric lt (<) at /usr/lib/perl5/site_perl/5.6.0/i386-linux/Unicode/String.pm line 242.
[Wed Oct 15 19:27:24 2003] userpanel-procmail: Use of uninitialized value in numeric lt (<) at /usr/lib/perl5/site_perl/5.6.0/i386-linux/Unicode/String.pm line 242.
I also get the all other errors you reported. So those errors are not critical to your system.

The bad news: You have another problem and must look elsewhere.

/Jens