OpenID Connector for XWiki produces Certificate Error

So, the problem is as follows:
We have a UCS server, with XWiki installed, which is supposed to authenticate the user via OpenID. After a lot of experimenting, the Login page of XWiki now redirects to the correct page of the OpenID Connector Provider, but that page throws an SSL_ERROR_RX_RECORD_TOO_LONG error.

Does anyone have any idea how to fix this issue?