QUESTION gs 3.3.13 on php7.0

[df8oe]

I have php6.5.32 and php7.0 installed on my production system Debian 9 (running ~30 different gs installations). When using 5.6.32 all gs installations work flawlessly, when using php7.0 I only get a "Error 500" and nothing listed in server logs on all gs installations. Other sites (with WIKIS, discussion groups, other CMS) do work also - but no gs installation. I am confused because there is NO message in server logs - I do not have any idea where to look at.... simplexml, rewite, gd and curl modules are installes and activated.

Greetings
DF8OE