Did you check /userlib for any old versions of the VirusScan4Mendix java libs. If there are two versions, remove the oldest one(s).
Check if virusscan4mendixHelper.java exists
Regenerate Java actions in Mendix Studio Pro
Clean your build
Check import statement in ScanFileDocument.java
Good point Tim! I had deleted (and backed up) the userlib folder before upgrading. Now there are very few lib files because they go in vendorlib. However, the VirusScan4Mendix files weren't in either directory so I copied them in from the backed up userlib and now it runs.
Many thanks!