I was plagued by this suddenly a few days ago, and now I think I’ve figured it out.
It began when I signed into myEbay and got:
“There was an error in your input. Please go back and ensure that all fields are properly filled in.”
Nothing I could do, over several days, including calling eBay support, got me around this issue. The best eBay support could come up with was “use the Canadian service” – which curiously worked.
A check of the web shows this problem isn’t rare.
It also shows that it’s a generic PHP reply, and while it happens on eBay most frequently, it isn’t limited to them. That got me thinking.
I went back to eBay.com (which had been giving me the errors) but instead of going to the sign-in page, I just entered “ebay.com” in the URL field… and bingo! I was in.
And as I guessed, I was also signed in!
I think the problem is simple: eBay failed to sign me out for some reason. The input error went away as soon as I signed out of the dot COM account, and everything is back to normal.
So, if this is hitting you, try just visiting the site, bypassing the sign-in page. See if you can sign OUT from there.