Ok now I get it.
First: you should never start 2 intial downloads on dependent objects at the same time. This is error prone... so please kindly wait for the initial download of BUPA_MAIN to be finished before starting IDL on BUPA_REL.
Then: if you load massive amount of data into ECC, I would recommend to stop the delta replication to avoid errors and (even more important) this will also avoid to slow down the performance of your systems significantly.
Plus: usually people creating program in ECC doesn't really care about the best place to write a "COMMIT" statement with a connected CRM system... and I can tell you this is where the error lies 99% of the time, because when you COMMIT a lot of stuff happens in the background to create Bdocs and send them to CRM. If the commit is not at the exact right place, some buffers will be cleared before the Bdoc can be created successfully... which will result in many errors in SMW01.
So let's start over: if you create a relationship manually in CRM (or ECC), does it flow successfully to the other system? Because if the answer if yes: check your Z program in ECC with a senior developer... or stop the delta replication while the program is running, and start a request load afterwards.
Best regards,
Nicolas.