Hi Alex,
As mentioned by Kul Vaibhav, while transferring the Customer to GTS , the system will check for valid data exists for the BP to whom you are transferring i.e., Address , Tax data etc., before updating in GTS. if any data is missing or incorrect then it will throw the error.
so, check whether all the data is maintained in ECC and try to retransfer the Customer to GTS.
is this happening in all your landscape of GTS? what about Vendors , Contact Persons, Materials. are they are transferring without any issues?
hope you need to debug to identify the issue .
Thanks,
Srinivas Y