Size of a request header field exceeds server limit

[eluser]Unknown[/eluser]
Just adding my two cents. Even though this is an old post it is near the top of Google with a "PayPal Error 400" search.

Had the same problem and tried an experiment since this seemed to be an intermittent problem. One commonality of all the people that complain is that has to do with site administration.

So I had just fixed my code (AGAIN) and tried the PayPal button. Error code 400 of course. So I logged out and tried it again from the same page (without reloading) and i DID NOT get the error - it worked like it should. So I logged back in as admin and tried it a third time and guess what - right - got the 400 error.

So this makes me think that being logged in as admin may have something to do with getting this error when working on a site with PayPal.