@PheasantRunAntiques: There are innumerable reasons for this particular error. One of the most common causes of "size of a request header field exceeds server limit" is too many cookies being sent in the header - and Etsy leaves lots of them. The problem may be intermittent as cookies are deleted or expire. This may be even more noticeable if you log in and out of multiple shops using the same browser.
According to other posts in this forum, another common reason for the "Bad Request" error seems to be that some browser extensions cause issues - Sale Samurai is often mentioned as a culprit.
