Error 502 on login page

Hello all. I installed my univention core and its working like a charm in as a xcp virtual machine. It hosts my owncloud and onlyoffice intalations. But I had a problema with my NFS, solved already, now my univention does not login anymore. It is linked with my AD server and after a couple minutes I get a error message: “An unknown error with status code 502 occurred while connecting to the server, please try again later.”, probably it does not connect to my AD anymore. What can I do? Thanks a lot.