Upgrade from LR 5.2.3 to LR 6.0.4.
During the upgrade, procedure "upgrade.v6_0_1.UpgradeDocumentLibrary" will add an entry of the most recent dlfileentry version in dlfileversion. Also the version will be converted from long to a string type.
Our DB contains a couple of entries in dlfileversion with version 1. In that case the upgrade will throw an exception. After removing all version = 1 entries in table dlfileversion the procedure executed fine and all missing version 1 entries will be re-created.