Solidworks Flow - "Advanced materials check"

[tinnit]

Has anyone else seen this error when trying to run a Flow simulation?

It reads:

SOLVER ABNORMALLY TERMINATED! Please contact the support service.
Warning: Preparing failed
Comment: Please check geometry with enabled option "Advanced materials check "

Ran Check Geometry with every option enabled - no errors.
Ran Interference Detection with coincidence enabled - only got the coincidence errors I expected to get (flow requires plugs to coincide).
Ran Check Entity with stringent enabled - no errors

I've remade this part twice and still no luck. Other parts run Flow just fine in the same directory and with identical conditions other than geometry.

The flow user's guide has this to say about "Advanced materials check":

Advanced materials check. Allows you to check solid bodies for which different solid materials are specified and detect invalid contacts between surfaces which are not in contact with fluid.

Other than that I can't find a single reference to this error in the documentation or online.