Error MSExchange Common 4999 on Exchange 2016 CU12

I have a client who is getting the following error at the event log: 

Watson report about to be sent for process id: 7256, with parameters: E12IIS, c-RTL-AMD64, 15.01.1713.008, M.Exchange.Pop3, unknown, M.E.P.C.R.d__404.MoveNext, System.NullReferenceException, 7748-dumptidset, unknown.
ErrorReportingEnabled: True

Client is using Exchnage 2016 CU12 ( Version 15.1 ‎(Build 1713.5)‎)

Not sure whats causing this error or how to fix it. Any input will be appreciated.

Thanks