My workflow which was working well so far has suddenly started giving me error related to approval status, the error message is 'Invalid approvalstatus reference key 11' I am trying to use the Set Field Value property on the ‘Approval Status’ as normal and I have no idea why it has suddenly started erroring out.

What record type are you running the workflow on? And what value are you trying to set in the Approval Status field?

Is that Status still Active.