I've never used Webmin before, but my first thought was that maybe the log file it's trying to write to is read only, or you don't have permission to access it.

It's just a guess, though I don't see why there would be a problem with writing to a log if it's on your hdd - that should only be a problem if it's on a read-only CD (or more specifically, a read-only dir on the CD).

Again, just a guess.

Good luck.