Segwit Address - bech32

I have some BTC and XCP locked in the Counterwallet on a BC1… address (segwit address, that is bech32-address).
If I try to send my funds to any remote address (both legacy and segwit), I get the following error:

Error making request to UNKNOWN/Default: JSON-RPC Error:
Type: Server error
Code: -32000
Message: Bad status code returned: ‘500’. result body: '{“result”:null,“error”:{“code”:-26,“message”:“64: non-mandatory-script-verify-flag (Signature must be zero for failed CHECK(MULTI)SIG operation)”},“id”:0} '.

I already wrote to the counterparty support, without answer.

Anybody could help me?

Alternatively, anybody knows how to extract the private key related to a segwit address?