Every time I come back to mantisbible.com, it logs me out. I have the preference to stay logged in set, but it doesn't work. (I am using the latest Safari for Tiger 10.4.11) Also, I can't post in Rich Text Editor. It just discards my posts. So I have learned to "Copy all" just in case so I can re-paste it. It takes a couple of tries for my posts to "stick" and only using Basic Text Box

Make sure your browser is accepting cookies -- that might explain the problem.

You might try using Firefox 3.  That's what I'm using and it works fine for me.