Hey @sbauch! The issue you describe is typically caused by an error in the EIP712 signature. Most likely, the payload you're signing does not match the one expected by the EIP712 verifier contract. You'll want to compare the getMetaTxTypeData function in the signer with the verify method in the MinimalForwarder, and make sure they are both operating on the same structured data.
That said, from a quick look, it'd seem that the incompatibility is in the domain name: you're using GSNv2 Forwarder in the signer and MinimalForwarder in the contract. Try changing the domain name to MinimalForwarder in the signer as well, and try again.
Please let me know if this is code you lifted from the workshop, so we can fix it so no one else runs into this! Or even better, if you can send a PR, it'd be greatly appreciated!