image rejection, similar image

Nothing.

First, it may also mean "similar assets in the database", even if the stock text for the refusal suggests otherwise.

Second: there are many reports that a lot of people are getting this error. So it seems to be a bug in the system. I would suggest contacting Adobe contributor support: https://helpx.adobe.com/stock/contributor/help/Need-Help-Contact-Us.html

They will answer with a copper plate text, but at least, when a lot of people are complaining, they may get aware of the issue.