Try running

rm -f /Library/caches/com.sophos.sav

I'm at home on my personal Mac but if I remember correctly that should fix your issue.

Thanks, @Snickasaurus but that did not do it. on-access scanner is still off after removing this file.

Are you using the Sophos Management Console or is this an unmanaged version?

This is an unmanaged version.

Oh well sadly I have to say I won't be much help then. I would suggest creating an account on the Sophos forum ( "community" ) and ask for help there. You can do so by clicking here and then clicking the blue silhouette in the top right corner to create an account.