I encountered this error when running unit test, after completing Service Update 10.0.39 (on dev environment), Build model, and sync database.

dashboard of D365 open still ok, no error , Service updated successfully.

I try to stop service SQL reporting, stop iis , refresh model, full build (with customn models), and sync database but it not work.
Message Error : Serialization version mismatch detect, make sure the runtime dlls are in sync with the deployed metadata. Version of file ‘209’. Version of dll ‘208’.

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dashboard of D365 open still ok, no error , Service updated successfully.

Thanks for your help

Solve issue about Serialization version mismatch detect, make sure the runtime dlls are in sync with the deployed metadata. Version of file ‘209’. Version of dll ‘208