I have the same issue. I installed the MOD and had to change the sever and cookie setting as per phpBB Server and Cookie settings
It was rewriting URLs OK but adding in the folder name twice i.e. mysite.co.uk/forum/forum/topic
I went away and came back and now I cant login. I tried uploading a cookie script to the forum root and changing my server and cookie settting back but it didnt help, also tried deleting the MOD elements in my root .htaccess and didnt help.
I can see my board index login page but just cant login. It doesnt return an error but just takes me back to the login page?
Can anyone help me?