Bitcore-wallet-client "Server response could not be verified"
The problem was that I was creating the transaction proposal and publishing it, after that I used the result from createTxProposal and passed it to signTxProposal.

turns out that the txProposal returned from createTxProposal doesn't have the property proposalSignature (which is filled when the transaction is published).

so, to solve this I had to get the transaction proposals (after publishing) and send the transaction fetched as argument to signTxProposal.