According to Special:Version, you have an extension:
"edit user page, Hook for editing the userpage via a formular,
by M.Boese"
If it's just user pages that this is happening to, chances are
it's something to do with that extension. Try uninstalling it
and seeing if the problem goes away. If it does, either do
without the extension, or contact the author.
I removed the extension and the user page works fine.
The problem was caused by the extension. I'll check the code and try to
fix it.
Thanks a lot.