Hello Emre,
1. Enable the view live log and check whether getting anything there.
2.Try to test your SAML configuration in local run, if you will get success then mostly it will work in higher instance. You will not be able to test in local, but you can configure and check able to setup with given IDP successfully, mostly times we saw error in local itself.
3.Mostly cases after updating SAML we need to update encryption also.
4.Check in userlib folder is there any duplicate jar file.
Has anyone allready fixed this? I got the same problems?