Using Chrome, the error message "Bad Request - Header Field Too Long" displays when a Clarity tab is open about a week. Tried using an incognito browser and it worked for a while but eventually received the same error.
In general this is a Chrome issue. and is caused by a corrupt cookie, please clear the bad cookie(s) and see if this helps:
1) Go to chrome://settings/cookies, or manually go to Settings->Advanced Settings->Privacy->Content->All Cookies and Site data). From there, you can search for cookies that match the Clarity URL you are having problems on. Finally, click "remove all" for the matching cookies.
2) Right click the lock in the address bar area (see picture below). Under cookies there is a link saying how many cookies are used. Click that link. Remove all cookies in there.
How to Clear Cookies for One Site in Google Chrome