Build is failing and getting error around `className` as an invalid prop

I am presently using frontity to build for a few months now. Out of the blue, a warning around className not being a prop appeared, and also, the view doesn’t rebuild. Need suggestions on how to fix this.

The error Prop className did not match. Server:

Please advise on how to go about fixing this.

Warm regards,
Himadhar H

Deleting node_modules, package.json file etc resolved rebuilding issue. But the warning needs to be fixed.