"disallowed key characters" mas OS Sierra 10.12

Try setting up another admin user account to see if the same problem continues. Also try the Safe Mode.

Safe Mode - About El Capitan
also Sierra

Safe Mode Startup – El Capitan. also Sierra

If it works in the Safe Mode, try running this program when booted normally and then copy and paste the output in a reply. The program was created by Etresoft, a frequent contributor. Please use copy and paste as screen shots can be hard to read. Click “Share Report” button in the toolbar, select “Copy to Clipboard” and then paste into a reply. This will show what is running on your computer. No personal information is shown.

Etrecheck – System Information

Usual cause is an error on the web site, usually with either reading or writing cookies though there can be some other causes. Could be a character in a username or something in the all-sorts-of-other-data that can be read from or written to a cookie. If you're using a bookmark to access the web site, try entering the web site directly into the address bar. Using a different browser might be a workaround, with different saved URLs and different cookies and such. Try clearing all your cookies in your browsers, try a manually-entered URL, then post the Etrecheck output on the off chance there's something odd in the local system configuration, and if none of that turns up a trigger then you'll likely want to check directly with the web site maintainers. (Yours is not the only recent case of this, too.)

Do download Chrome and Firefox and try them. Access is more important than what browser you use.

Do you have this problem with chrome or Firefox?

what is the url?

Works fine in High Sierra with Safari, Firefox and Chrome. Which version of Safari are you runnng?

Just checked Sierra, works fine there as well.