I had a working hybrid setup with one 2013 server onpremise last week. Then I had this great (read stupid) idea to install a fresh 2016 CU5 server onpremise and remove the 2013 server from the domain, just for easy administration as all mailboyes already moved to exchange online.
Now I have trouble accessing the EAC in a very weird way I do not understand, nor get any meaningfull error messages which would help me fix the problem.
If I try to login to my EAC with a url which just end on /ecp I get a 500 webserver error and are unable to access the EAC.
Luckily for my, I have a bookmark ending the url with /?ExchClientVer=15. If I use that one, I can login just fine. Although the Office365 button works and gets me to Exchange Online, the Enterprise button does not get me back, but gives me the 500 instead. Reclicking my bookmark gets me back to my onpremise server.