Well, I haven't seen it anymore since I disabled the adblockers, so it looks like this was indeed the cause.
There is one other possibility, but that is rather far fetched. I installed a new Sitecom dualband router about a week ago, and the errors may have started around that same time (no way to be sure, however). Today, around the same time that I disabled the adblockers, I updated the firmware in the router. However, that would mean that somehow the router firmware caused this problem, and only 'lightroomqueen.com/community' was affected of all the websites I visit and all the other things I do on the internet. It's not 100% impossible though, because I don't think I participated in any other forums that use the same forum software.
Anyway, it does seem that the problem is over, because I can type this message too (on my Mac this time) without getting an error.