Sadly, they didn't tell me who it was, although I did ask multiple times :-(
It seemed to have been a grand sweep of artwork that they blocked for this person/company. Redbubble wasn't very helpful either, I just had to contest the claim, wait for a couple of weeks to hear back from them (which we didn't), after which they re-instated the artwork.
No matter how many times I asked them "How can someone trademark black and white stripes?", they never gave any meaningful answer. Very frustrating.

Happy to see you pop up here! How have you been? 🤗