Mathoid has been overwhelmed with erroneous requests from the Math extension. A sample of MW exception log entries can be found here. It seems most exceptions are generated only for a handful of pages:
At the same time, Mathoid's production logs show repetitive 400 errors. See P2422 for a sample of them.
I have tried to issue requests to a local Mathoid instance for all of the <math> objects found in the first three pages cited above, but all of them passed the check.