I had these problems as well. They were caused by corrupted cookies and the best way of solving them is to delete the cookies related to your forum. After that, the problem should be solved.
As regards logging in, I noticed that it is often best to login on the forum and then switch over to the wiki. Logging in on the Wiki page does not always work.
I put together instructions on how to do this on the
main page of my own Wiki which might help you, too.
I hope this helps