Share all details related to your problem, including any error messages you may have received.
I have a work-item forwarded to a user A for an approval. Anytime user A tries to approve he gets a warning stating " User does not have access to this work-item".
Is this warning due to any of the reasons like: the approver does not have proper SPRight /Capability or being an end user (not being manager)?
This is not the right way to do if you update the owner in debug page , did you revert from debug and forward to same user from UI and still the same issue?