Today counterparty releases version 9.56.1 which includes a hotfix for an uncaught exception which would cause a Counterparty server to stop operating normally.
All users are encouraged to upgrade as soon as possible.
For those running a federated node with no front-end:
fednode update counterparty counterparty-testnet
fednode rebuild counterparty counterparty-testnet
For standalone / manual installations:
pip install --upgrade counterparty-lib==9.56.1