I registered on your site, but it keeps refreshing itself, but it finally loaded.

First try this.

Admin CP >> Maintenance >> Repair / Optimize Tables

Check All in the top left, then continue towards the bottom.

But, it could also be a cookie problem.

If it is a cookie problem, then you can reset it with tools.php. You can locate that file from your vbulletin files in the do_not_upload folder. You need to place it into the admincp.