Error when commenting

Hello there,

Apologies for the late response.

Did you make any changes directly on the theme files?
This issue is similar to the discussion here:
https://stackoverflow.com/questions/50561039/catchable-fatal-error-object-of-class-wp-error-could-not-be-converted-to-string

You can try to follow its solution and see if the same works for you.

Let us know how it goes.