So apparently when I post a new post (like this one), the spoiler tag works again:
Spoiler
Hello, I am a spoiler
(I didn't touch any other threads, tho.)
My guess: This was caused by some update gone wrong that somehow triggered a feature/bug (???) in phpBB that somehow forces phpBB to save the "[ spoiler ]" tag as plain text (without turning it into an actual spoiler element). Or maybe this is some weird hidden escaping going on. But I have no idea how phpBB saves posts internally, so this is a 100% wild guess, don't trust me! Or maybe this is even an outright bug in phpBB because this behavior makes zero sense. But to be honest, I'm grasping at straws right now, sorry. :-(
But looking at how re-saving my posts fixes things, that could be a pointer to the solution? Is it possible to force phpBB to "re-save" all posts or something like that? (after doing a backup, of course :P)