I am having a similar issue with my eBay integration with my MyPostBusiness account.  Apparently there is an issue that they are trying to resolve.  I navigated to the eBay Help page for Labels, addressing and packaging tips and it states at the bottom with a red exclamation mark:  "If you're having trouble using certain areas of our site, please try again later.  We're looking into it now and will have it fixed soon."

I don't know if that is always there.

I cannot find anything else about any issue except your post.

Hopefully whatever it is will get resolved soon.

Not just you. I can't connect my ebay account to mypost business.

"Oops! Looks like we had some trouble connecting to your eBay account. Please try again"

on eBay postage there's a message that says "Please be aware that Australia Post is experiencing issues at the moment. We apologise for any inconvenience and if affected, ask that you try again later."