We need more information from you.
What extensions are you using? When did you notice this problem (did you change a setting or add a new ext)?
Did you hard refresh your browser on the page you received the error?
Did you run php flarum cache:clear
via terminal in your Flarum root folder?
And can you change 'debug' => false,
to true in your config.php so the next time the error occurs, you can click the DEBUG link and paste in the error code that pops up?