Sorry that I can't be more specific atm, but I haven't been able to find a reproducible case. I'm waiting on someone who knows how to debug this stuff to show up in #twiki, so I can figure out how to put in print statements to try and narrow things down. Sorry to be so vague.
This is in 4.0.1, without mod_perl.
At least tell us what login manager you are using!
So sorry. Using form manager.
Hasn't happened since. So i'm discarding this.