Q: "disallowed key characters" on Yosemite 10.10

That's an error on the web server. There's probably nothing you can do except to notify the server administrator, but you can try the following step.

From the Safari menu bar, select

          Safari ▹ Preferences... ▹ Privacy ▹ Remove All Website Data...

and confirm. Test.

IN Chrome try clearing your cache and cookies. It worked for me.

Clearing cache, closing Safari, and rebooting my Mac didn't work for me. Any other ideas would be super helpful.

You might want to start your own post since this one is from last year. A new post would be much more visible. You can link to this one.