Hello philhguk,
I have never encountered that problem exporting from AR5 Nor do I think, judging from a lack of forum comments here, have many others encountered it!
What file format are you trying to export to when you see the message?

You may also want to contact support directly about it here: [email protected]
With as much info as you can e.g. system OS & version, ArtRage version and exact steps to reproduce the problem.