NAV 2015 Word Report IOException

Anyone ever encountered the following? Any ideas?

Whenever there is any C/AL error (TESTFIELD or whatever) in a Report that uses Word layout, the RTC shows this instead of the expected error message:

A transaction must be started before changes can be made to the database.

.. and this is written in the Event Log ..

>>
Server instance: XXX
Tenant ID: default
Session type: RoleTailoredClient
Session ID: 25
User:
Type: System.IO.IOException
Message: The process cannot access the file 'C:\Windows\ServiceProfiles\NetworkService\AppData\Local\Temp\tmp1B43.tmp' because it is being used by another process.
StackTrace: at Microsoft.Dynamics.Nav.Runtime.NavApplicationMethod.InvokeMethod(ITreeObject obj, String methodName, Object[] args)
Source: Microsoft.Dynamics.Nav.Ncl
HResult: -2146232800
<<

This is NAV 2015 CU1, and running NST with other credentials does not change anything.

Switch back to RDLC, and it's fine.