The sim, maybe even the entire grid, still runs OpenSim 0.8.2.1, an old stable version that's superseded by four newer stable versions already. 0.8.2.1 was the very first version with BoM support, but only with very basic BoM support.
OpenSim 0.8.2.1 does not know universal layers yet, though. It must have been around 0.9.1.1 which introduced full BoM support when universal layers were introduced.
This message comes when your avatar wears a universal layer. You can tell them by the symbol, three red rectangles. Ruth2 v4 uses them for optional nail polish, and I think EvoX heads use them for something else, I don't know what, I've got zero experience with EvoX.
If you take the universal layer off, you should be able to enter the sim, regardless of body or head.
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