The rest of that paragraph explains it? $mysidia->user->register(); (which exists in classes/class_visitor.php so I had you make sure no edits were made there) is called first. Simple as that? All it does is process the form and should not output any data.

Who else has an integrated forum? I do and I don't have this issue.

If I recall correctly, mysgardia.com, which I registered yesterday and Distortion (in second post this thread) says they also have the problem...

And what version of MyBB are you trying to install? Could be relevant.

Well, I'm not sure, HoF installed it for me.

So, @Hall of Famer ?

Yeah I remember, I did install it for her and integrated her site and forum a while ago. It clearly was working before, as I did a quick search in her site and forum databases in phpmyadmin. As you can see from the two screenshots, the users tables in site and forum databases start to diverge after user ID 25(until ID 24 everything was in sync), where the site has user 'test5' and the forum has user 'test7'. Everything has been out of sync after user ID 25:

http://oi64.tinypic.com/2f07fad.jpg http://oi68.tinypic.com/2q9031w.jpg
Site User Table Forum User Table

So what I conclude so far is that something happened at the time when she created the test user 5 and it was only inserted to adoptables site database but not forum database. It can be several reasons, and these are the questions that need to be answered:

1. Did she modify the script in some way that broke forum integration, at about the time account test 5 was created?
2. Was the site down/offline right after she created the account test5, so the forum user aint created properly?
3. Did anyone try to register on the forum, and not on the site? Its necessary to turn off forum registration.

I turned off forum integration when I reached user TeddyBear. I didn't want to have an error showing and was considering whether I wanted it or not. But now I wanted it again.

I see, so you temporarily turned off forum integration after user TeddyBear signed up? And when you registered account test5 you didnt have forum integration back on?

Anyway I did see that you posted this thread back on Dec 18th, and user TeddyBear signed up much later, which was Dec 23-24th. This means that forum integration was working for you, although you got some strange warning messages. These warning messages dont make your site malfunction though, just its a bit annoying I guess.

Now if you want to enable forum integration again, it will be very tricky. Its doable, just manually sync the IDs of your users from adoptables and forum database. However, this wont integrate accounts that are already out of sync, but it will make future registered users in sync from your site and your forum.

Yes, I noticed, but I don't really mind because the accounts are all pretty much blank. Can anyone help with the errors?

I googled the error and people said to check if there were any spaces before <?php. There was a space in config_forums.php but that doesn't do anything...

I already explained it can be caused by anything being rendered to the page. With a default install and a forum properly installed, this does not happen. An error about your forum's configuration that you cannot see is being thrown, rendering an error to the page, resulting in a header error. Specifically one being caused when it is unable to set a cookie because cookie information does not exist.

1) Are the files I mentioned earlier all brand new with no changes? Yes/No?
2) Can you log in directly to your forum's admin area? (Note that this is not the same as logging into your site). Are you able to edit the forum's settings? Yes/No?

It's really not clear to me that you've tried anything I've suggested, so I can't help any more than I have. I really need confirmation feedback on all the instructions I've given so I can help pinpoint the problem.