2015 Utility Dry Van Trailer WHEELS LOCK UP

[MPDiagnostics]

I would say one of the first places to start is to pull any codes (active or inactive) you can from the trailer ABS. You did mention that there was an ABS light present earlier. The service brakes are probably controlled from the electronic ABS Valve unit. This information is necessary to point yourself in the right direction.

If the driver says that everything was fine for a little while but then came up again it seems like an electrical issue (sometimes particularly in rainy conditions).

If the probably is currently persisting check and see if the ABS light is on.

I have also found corrosion on many ABS connectors so its worth opening those connectors and inspecting for any corrosion.