Device becomes undiscooverable for no apparent reason

After a stable period of monitoring a NAS4Free host running for at least 6 weeks. Observium just failed to stop monitoring this host.

Gui fails on attempting to force a device discovery as shown in the attachment.

Pings and SNMP walks from the observium server itself work perfectly but the poller and discover scripts fail with an error.

No updates of changes had been made to the observium installation. which has been working faultlessly for >2 months.

I have tried rebooting observium and the remote host, with no change in behavour.