The Twitter link shows a picture taken at a race event, where it says on the door: wedstrijd secretariaat, meaning secretariat competition in English. It's two words, so the first modifies the second (adjective) rather than forming a compound noun, thus some wedstrijd (competition) of the secretariat seems to be held there. Writing wedstrijdsecretariaat as one word makes it a compound noun and translates as competition secretariat which is (presumably? :D) what was meant. Ha-ha! Germanic humor, I guess. (I really enjoy them at least, since it really is what people wrote and they don't even realize it. Probably ties into pentesting, where I also exploit what people incorrectly wrote?)

> Sometimes the addition or lack of a space can change something quite a lot, so you can't just insert them because it's convenient.

Correct, but note that hyphens between the parts are always legal if you think it's more readable.

For example meervoudigepersoonlijkheidsstoornisbehandeling was not hard to get for me but then the ...behandelaarsopleiding variant is really stretching the possibilities and I'd definitely start to hyphenate there, also because it's a bit of a false start (it's an education, but you're starting off with a disorder and then segueing into treatment and then again veering off into it being an education that you're describing -- it's a bit like "The old man the boat." in English: a garden-path sentence or an intuinzin which starts off making you think it's one thing and then continues in a way that forces you to reevaluate it).

Also, if you have a reason why you didn't put an "s" between behandelaar and opleiding I'd be interested! It feels to me like there should be one but I don't know the rule.