We’ve seen this error on WordPress websites using the WP Super Cache plugin.
To resolve this please try clearing your WP Super Cache and then reopening your web page in your web browser.
How to fix WordPress error ‘Warning: session_start(): The session id is too long’
My WordPress web page is giving me the following error - How to fix Warning: session_start(): The session id is too long or contains illegal characters, valid characters are a-z, A-Z, 0-9 and '-,'