OK – tracked it down – turns out cache is written to the wordpress installations root directory, which then must have write permissions.
I changed owner to www-data:www-data – but don’t really know if that’s a good idea.
The plugin uses WordPress MVC framework which creates the following file structure at the root of your wordpress setup:
/wpmvc/logs
/wpmvc/files
If you wish you can turn of write permissions of the root directory and manually create the /wpmvc folder and give write permissions to that folder instead. That should work to.