501 error: service not implemented

sudden site errors....why? how to fix?

[chasing something]

after running smoothly for over a year, one of my sites is suddenly sending some users a 501 error (service not implemented).
the whole site is plain HTML, plus adsense (JS).

my host doesn't know what to make of it, and either do i...lol.

is this a server thing or a site thing?

can anyone help me figure out how to fix it?

thanks!

[Span]

Maybe it's not the server or the site, but those users computers?

[support.microsoft.com ]