Hi Barry,
I checked the note 1781003 (already implemented) and 1789801 (not yet implemented, because error looks different).
After that, I've executed the report ESH_ADM_INDEX_ALL_SC, which deletes all settings from the CRM database with SQL. For sure, I have set the TREX Destination again in the customizing.
At last, I was able to create the connector without the error!
Many thanks!
Dieter