Admin CP Throws Back to Login
 

Recently I'm receiving this error, which I've never come across before and I can't seem to find in searching these forums. I've installed vB plenty of times, haven't done anything much different.

Whenever I click to log in to the Admin CP, enter my information, and log in, it will go to the screen where it says "Thank you for logging in! You will now be redirected." or similar. Then it throws me back to the login screen for the ACP.

It is not a matter of lack of admin permissions, since I'm user 1 and have all the superadmin privileges allotted by the config file. It also is not an account problem since I can log in to the forums just fine.

The only things I could think of are some lack of the admincp folder improperly uploading or similar, or something to do with using MySQLi database type(with PHP/MySQL 5). Something odd I noticed, which I'm not sure has to do with the database type, is an addition to the URL which has not been present on all of my other installations.

mysite.com/admincp/index.php?

Becomes the same except with an S = and string of a lot of characters at the end(session? Whenever I add it to the URL here it turns invisible).

The same is present on all other pages. Has anyone else experienced this or know what might be causing it? I have access to the databases and the overall server administration, but am only versed in PHP enough to follow instructions.