Thanks for your reply Sean and the link.
What happens when one of the feature classes is versioned and the other is not? In this scenario I switch off versioning as you describe and I get the warning message: incorrect edit version type.
By default exports of data are not versioned unless one runs a toolbox to version them or they do the versioning in ArcCatalogue, remembering not to add the data into ArcMap first or it will be locked until the ArcMap application is quit.
Yes. Installed 10 SP5 on a client machine and this fixed the issue.
The Bug IS BACK! Exporting fc as table and doing the join worked though. I'm in ArcMap 10.5.1, using SQL Server 2012 db.