Thanks for the help, that was one part of the solution.

In addition to that, the kkp-directory under map_cache was created apparently with such access rights after my command line testing that the apache "www-data" user did not have rights to write to that directory. I gave www-data the rights to map_cache using chown, then deleted the contents of map_cache and then it started to work. Which was nice.

So, thanks a thousand for this. Now it's time to backup my precious Finnish TopoMaps cache and start playing with Mappero....