Hi,
I just noticed I was on an old forum version, 4.5.3. I've upgraded to 5.0.0 and I get:

A critical error has occurred.
Object reference not set to an instance of an object.

whenever I try to view a thread. So my forum is broken!!! I've put a message on the page to let people know, but I have no idea what do do now.

I can't rollback as I have the newer version installed. I don't want to remove and re-install as I imagine this might lose all the current data?

Can someone please help?

It's a permissions thing somewhere I think. I posted as superuser, and that thread is viewable by anyone. I posted as admin or a registered user, and that post is not viewable.

I've the some problem too!

But in a a new and clean DNN installation5.5.1. Forum 5.5.0
I'm looking for the solution. I find the same situation you describe in your posts.
I can only view post from superuser and not members. If I give to a member superuser right it work properly.
SQL profiler whow only one difference:
Opening the post of superuser exec a store before the Forum_Post_GetAll:
exec dbo.GetTermsByContent @ContentItemID=xxxx

that opening a member post not exec.
This is the only trace difference.