By chance I discovered that files below 10Kb size could be uploaded and from there I managed to find the cure. Unfortunately without really knowing what is going wrong:
Check if you have port option OPTIONS_FILE_SET+=AP2FILTER (Apache 2 Filter) on for mod_php56, but PHP configured the usual way as a handler ( AddType application/x-httpd-php .php ).
Disable AP2FILTER, recompile, deinstall and reinstall -> WORKS.