This is the weirdest problem of my life, and can't even Google it. It's happening on an Apache powered website written in PHP, which uses mod_rewrite (but that's not the issue, I tried removing the .htaccess file, problem still exists).
If I have a query string that looks exactly or similar to: =/id
I get an 501 response:
Method Not Implemented
GET to / not supported.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
I never written such error page, never sent an 501 response, don't have the slightest clue where this thing is coming from. Originally I had a long URL giving me a similar error, but I stripped down to this little snippet above. If I remove or change any character, the error's gone.
If that helps: my website is commentards.net, and the original URL was an openid login request which looks like this:
http://commentards.net/q/user/auth?openid_identifier=https://www.google.com/accounts/o8/id
from which the query string is:
?openid_identifier=https://www.google.com/accounts/o8/Fid