The old boards used a different spoiler tag plugin. The new one still uses the same
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Old posts with spoiler tags will be broken until you edit the post, add a new spoiler tag, and save.
Kind of a pain for users with long-running threads they maintain, I know, but at least it's fixable. I'll make updates to posts that I come across with old/broken spoiler tags, so if you see an "edited by wermy" message here and there that's probably why.
The new spoiler UI is a little nicer, too: