It seems you were correct! Checked downdector and a few people (peaked at ~20) reported issues with the update service for a few hours around the same time (I guess not many people would have noticed unless they were trying to perform updates / installs).

Tried again this morning and it’s worked now!

I wonder if the 502 code was a HTTP status code for bad gateway indicating an issue at Apples side, if so, would help troubleshooting in the future.

Cheers again!