Can't upgrade to IPB 4

I upgraded my forum to IPB4 a while ago but we weren't satisfied because of all the bugs IPB4 had. Because of that we decided to downgrade to IPB 3.4.X with a backup. (database and all) But now we wanted to try to upgrade again to IPB4 but unfortunately the upgrader isn't working as it should work. Please look at these images.

When we click on the convert url, this happens:

But when you click ''proceed'' it will redirect you to the log-in page of the upgrader and that happens all over again. Because of that we wanted to try something else we tryed to upgrade the database to UTF 8 with this script: https://community.in...base-converter/. It solved the problem above, but then this problem happens:

If you click continue it will give even more SQL errors.

Please help. Thanks in advance.