[Macsee]

I sent it to you by PM, ForceHSS. Thanks in advance for any help.

--------------- Added [DATE]1327060665[/DATE] at [TIME]1327060665[/TIME] ---------------

I managed to exchange a quick couple of texts with the developer. He's not too familiar with vB. Anyway, he's said this:

the login script appears to be redirecting to the forum.php script, not back to the index.php - forum home page - as I believe it should. That said, there's enough going on within login.php that I do not want to edit it without being a LOT smarter about vBulletin!
...
The login script sends the visitor to the $vbulletin->options['forumhome'] . '.php' variable (not index.php) so, if there is a place to change the forumhome value, check that.

Does that help point the finger anywhere?

He also said

I don't know enough to alter anything in the user.php script area but suggest that whatever was tweaked in there caused the problem.

I didn't "alter" anything in user.php. I didn't even realise such a file existed. Would some change I made while logged into admincp have inadvertently changed this user.php file?

--------------- Added [DATE]1327084668[/DATE] at [TIME]1327084668[/TIME] ---------------

I've sorted it!!

In case it helps anyone else, I'll tell you what finally did the trick. I was pulling my hair out trying all kinds of things and even several vB experts weren't able to say what causes this problem i.e. when you can login to admincp but not into the forums.

It turned out to be
AdminCP> Options > Site Name / URL / Contact Details

I had to remove all mentions of the www in there as the htaccess redirects all pages to non-www URIs. That solved it.

Thanks to everyone for help and for offers of help.