ETS 5 -> ETS 6 Import failed - An item with the same key has already been added

I have a problem with importing a ETS 5 project into ETS 6, it works to open i ETS 5, but I get the error message "An item with the same key has already been added".

Anyone got any tips for this, have tried to import as a ETS archive project, changed language, installed previous version of ETS 6, but I still get the same error.

The project opens fine in ETS 5, but I don't have license for that myself.