"An error has occurred while using translation provider ...: Object reference not set to an instance of an object".

I have an issue with one TM only that triggers this error message: "An error has occurred while using translation provider ...: Object reference not set to an instance of an object".

I have updated the TM, I have created a new project, I have reindexed the TM, I have removed it from the project and added it again and closing and re-opening Trados and still no luck. 

This is only happening with this one TM and in a normal project (not Groupshare).

This is a long-standing client and I obviously would like to avoid starting the TM from scratch.

The TM is added to the "All language pairs" tab.

Any advice?

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