I have opened thread with similar issues. I think I found out what is going on after weeks of game going worse and worse with freezes. I saw your thread in Steam ESO forum, and that there are others with the similar problems.
Updated post below.
@IronManiden
Open Resource Monitor (resmon.exe) in Windows.
You have to monitor eso64.exe. Res. Monitor should obviously run all the time.
When game freezes, Alt-tab and check resmon.
eso64.exe constantly communicates with megaserver, in my case PC EU even when you idle on log in screen and main (characters) menu which is normal behaviour.I guess you're on PC EU, too as you wrote in German.
When freeze happens, there are ZERO bytes sent/received between eso64.exe and the ESO megaserver.
After some time, sometimes even over a minute, eso64.exe suddenly receives a lot of data, and you get notification from game that action happened. I literally just wait to see data received, and go back to unfrozen game.
Please do this, and report back!