Lotus Protector / Notes Integration Password Issues

We can consistently log into the the Protector user interface (https://relay.domain.com:4443) using our email address (from Domino LDAP), but the Notes integration doesn’t work as well. I’ve had a work a few times, but then it stops again with the message:

User xxx failed to log in.

The protector server seems to be temporarily unavailable. Please try again later.

How can we get this to work consistently?

Thanks

Steve