For several days, I was getting the 400 Bad Request Error Message most of the time when trying to open a Qualtrics survey link in a HIT (made no difference how I clicked or whether I used copy/paste):

"Bad Request

Your browser sent a request that this server could not understand.
Size of a request header field exceeds server limit.



The solution turned out to be ridiculously simple; just delete the cookies from the site with the problem.
Here is the link to the very easy tutorial which helped me fix the problem (I'm using Chrome, but the tutorial is for other browsers, too):

HUGE thanks to our Marvelous Manzician for the troubleshooting assistance!