That error is usually caused by signing the RAW Transaction with the incorrect private key or it's not fully signed yet (
coib.in will show a result even if it's not signed by the key you've provided).
Since the tool is coinb.in, you should use the "
verify" tab first before attempting to broadcast the transaction.
A ready-to-broadcast successfully signed Multisig Txn will show: a check symbol with the number of signatures under "
Signed?".
You can also check the outputs(s) to see where the transaction is sending coins to (
see if you're actually receiving bitcoins with that transaction).
However, that setup where you need to sign a MultiSig transaction in order to get a refund is sketchy unless explained why, you two are not the first to mention that.
I mean, why do you need to co-sign? Is their "
refund MultiSig address" co-created with your public key? This doesn't make sense.
Here's another similar issue with OP sharing his discussion with the 'service':
/index.php?topic=5387764.msg59399757