The only way that I can set around it for now if to go into Host settings and restart the application.

Onseveral occassions I actually had to log out and back in again afterwards

I've had this error with every version except 3.1.1 always with forums and galleries.

I would have expected it to be fixed by now really but it still hasn't. I'm now on 4.4.1 and still getting it (although the only place I've experienced it on 4.4.1 is when trying to post a new thread in the forum module).

Because of this I'm having to run 2.1.2 side by side with the later versions of DNN and I'm actually past the point of caring anymore. It's impossible to run and I don't have access to Visual Studio to change the code as suggested elsewhere. Before anyone else asks, yes I've switched caching to heavy and memory. If there's anything else I've missed I'd love to know because I've thought about ditching the whole lot on more than one occaision, which would be a shame because I absolutely love the software, it just will not work for me though :(