I first encountered this error about a week ago, while trying to set up a new purchase item for an existent app. Given the recommendation to try again later, I assumed it's a transient error and I ignored it.
About a week later I tried again, with the same result:
Hmm, now this doesn't look any more like a transient error, unless a really unlikely coincidence it at play here. In any case I alerted Apple. Then I figured I should also try with a different browser, and sure enough it works fine with Safari.
So if you're getting this too, try switching to Safari and you may be able to work around it.