Firefox is now able to use HTTPS for minecraft-nl.gamepedia.com, but unfortunately not (yet) for the other Minecraft Wikis (also decided to test faeria.gamepedia.com, no HTTPS connection either). Should've tested them all when I wrote the previous comment... Also, due to the fix to let Firefox use HTTPS on minecraft-nl (and other Wikis), Chrome decided it doesn't want to use HTTPS anymore (or at least, I assume so).
FYI: I don't want HTTPS right now, I just want to let you know about the current situation. It's better to have it a tiny bit later than expected with less bugs, than having it as soon as possible with a lot of bugs.
EDIT: Sometimes the siderail with ads is still visible, while I'm a PRO. Is this intended or another bug?
Another edit: The Wikis are able to use HTTPS. It was an interwiki-related issue (instead of //www.wikiname.gamepedia.com it was http://www.wikiname.gamepedia.com). The issue with the siderail is also fixed when I changed the interwiki.