Hi there,
the easiest fix is to use a different map layer - you are using mapnik.de, and by the time of release of the current plugin version they didn't have https-enabled tile servers, so the plugin tries to load the tiles via http..
You could either use the original Mapnik layer or e.g. OpenTopoMap, they both have https support enabled in the plugin. I'd recommend OpenTopoMap, imho it is the best looking map layer for hike & bike trails in europe.
Hth & cheers, Frank