Now I have tried it again: Yes, you are right. The problem occurs when I downgrade from 4104 to 4062. It persists when I upgrade back to 4104 again. (4062 up to 4104 ok - down to 4062 unknown - up to 4104 still unknown)
Now I can upgrade and downgrade as I like; the plugins are recognized again when I perform a full rescan of the VST folders in any version.