SX1 to SX3 Master becomes BUS1 plug-ins can not be found

On XP when opening old tracks created on SX 1 on SX 3.1 I get a warning message: the various (native Cubase) plug-ins on the Master inserts can not be found.
The reason seems to be that the main stereo output named “master” in SX 1 is renamed “BUS 1” by SX3.
All the other individual channels load their inserted plug-ins correctly.

I’ve tried renaming the bus but that doesn’t “reload” the track, so the plug-ins are still not loaded.
When reloading the song, “master” becomes “BUS 1” again and the main output stays without any plug-ins

Any Idea on how to get “bus 1” to be considered as “master”, so that the plug-ins load successfully?