I get this exception when attempting to connect to a secure websocket server on an address other than the addresses specified in the certificate (i.e. cert wildcard for *.acme.co.uk and connecting to PC1110 as opposed to PC1110.acme.co.uk)

Changing the connection to connect to an address which is covered by the certificate (i.e. PC1110.acme.co.uk in this example) fixes the issue.