That is suck... I haven't tried to edit my own, but I'll look at that. "My Posts" works on my own website, but I'll look at this one.
As a side note, they (DotNetNuke) are releasing a MAJOR new release of the forums package in the next few weeks which should resolve a lot of the issues. I hope anyway.
*EDIT* -- I can edit my own post (as illustrated here), and I have fixed the "My Posts" link. Please try to edit your own post (do you not see an "edit" button next to your posts?) and if you get an error, let me know.