I'm afraid I'm not technical enough to know whether this
is a problem with Logrotate itself or the webmin interface
however, here is the problem description.
Set up a logrotate job.
Use the "delay compression till next cycle" option
Use the "email log file before deleting: Yes" option
Use the "Log file to e-mail, Newest" option.
This should mean that you get an uncompressed logfile,
say logfile.log.1 e-mailed to whatever address you
specify. Unfortunately the job appears to be looking for a
logfile.log.1.gz where it attaches to the e-mail and
therefore results in an error.