According to the Signable status page, they have implemented a fix and are monitoring. Our testing shows it’s currently working again.

The digital signing service we use - Signable -is experiencing website issues. This will affect when signing a document or submitting a tenancy for digital signing. We will closely monitor the situation and update when their service runs again. We have spoken with the support, and they know the issue. You can check their status at https://status.signable.co.uk.