I am getting this error too. To be exact:
A critical error has occurred.
A control cannot modify its parents' control collections.
I am running Windows 2000, IE6, and have DNN 3.1.installed with SQL server (via GoDaddy)
Some things I have noticed:
1) Host/Admin never get this error.
2) If I create additional accounts (from the same computer), they get the same error.
3) Modifying permissions and page locations does not correct the problem.
4) Deleting IE files/cookies/cache does not correct the problem.
5) Creating new Forum modules/forum groups/discussions does not correct the problem.
The only thing that seems to help is time or a virgin installation.