These errors are intermittent. I do a daily hot backup but on Sundays I do a cold backup which involves shutting down the database and bringing it back online. Because we're using Oracle Failsafe the database is taken offline by using FSCMD.exe commands and then brought back online.
The first script is scheduled for 18:15 which shuts the db down and then copies all the files to a directory. Then at 18:25 another scheduled script brings the resource back online. What puzzles me is that these have been working for months without errors but recently there were problems with the network config etc and also the event log report the node losing communication with our server. A fix has been scheduled for this weekend so I'll monitor this and see if this will resolve our issue.