Came across this error in dev26 so I updated to dev26b but the isuse is still there. I'm running 1 version behind the latest Spigot release.
I have exactly the same problem with dev26. I downgraded to dev25 and everything was normal again.
Ah, this kinda makes sense. Not much, but still starting point for debugging...
This is likely fixed, assuming it was caused by {@prefix} containing invalid tag.
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I have exactly the same problem with dev26. I downgraded to dev25 and everything was normal again.