Spectrum syntax error, unexpected token "\"

Ho.

As fist step as Joomla Investigator - enable that:

Then check from where you have error, which file(s) is on table location 1 and 2

Also that message doesn't help :/ too short !!

Switch to PHP 8.0 and test again.

Have you updated all extensions and template to new version?
If you are not sure, check one by one.