I think I found the culprit but I think it may useful if I document what I did:
I created a new folder "plugins-off" in parallel to "plugins" in the "lib" folder (resides on the same level as the web root) - then I moved all folders of plugins that I knew were not there in a default installation to "plugins-off".
As soon as I could reload the page I knew I had isolated to cause. From there, I reactivated plugin by plugin.
In my case it had been the plugin "embed".