I apologize for not replying sooner. I thought I was watching the thread, but no notification coming.
This mod will remove old/duplicate session from the user, only if that user is logging in. This mod is not for doing session expire or session log-out, because phpBB is already doing that.
Probably your user is using their browser's cache to click back etc, but if they are logged out (their session removed) then they can't do anything else. To avoid this, maybe you can put a 'pragma no cache' thingy on your page so that users will not be able to see their old cache for your page.
As for setting the index page to non-default phpbb forum index, it should be already available in the mod database or in the forum.
I tried to duplicate your error message but can't. I installed a fresh 2.0.17 (no mod) and applied this mod, and everything works as advertised.
Basically, my mod will remove all session for that user, when the user is trying to create a new session. Thus, all old sessions will be removed, then the original phpBB code will create the session.
The sql are doing as follow:
1. delete all session for this user [this is what my mod doing]
2. update the session for this user [this is phpbb original code]
3. if cannot update, then create new session [this is phpbb original code]
My mod is not changing anything else. It is a run-and-that's-it code, which means after the end of my mod it will not be seen again. As you can see, I am only using the $sql variable to do the query, which then will be overwritten by the next query.
Also, there is a more descriptive error message in place, if something is wrong with my mod: "Error in removing duplicate sessions".
So, my guess is maybe you haven't put my mod fully (something changed), or your problem was coming from someone else's mod.
You can try to remove my mod (it's not crucial, so it will not break phpbb) and see if the problem persist. If so, then double check if the mod entered is complete.