Tried to clone an old site to a newly created one. I restarted the cloning 3 times until done it right. The error_log of the new site is getting filled with tons of errors (mine got 65MB of them) unless I recreated the new site, activated the same plugins like the source site and then started to clone it..
At some stage I even got error 500, not sure why but in console log all admin-ajax.php calls were returning error 500. Only activating the plugins solved the issue.