Thank you to all those who commented. At the end, it turned out that the root cause of my problem was a mismatch between the firmware on the MultiPlus (after upgrade) and what CErbo GX thought the firmware version is. We fixed it by re-upgrading the Multi to the latest firmware (552), deleting the (old) Multi from the VRM and rebooting the Cerbo GX twice, followed by rediscovery of the VE.bus devices - which after quite a long time (circa 20 minutes) resulted in updating the Multi parameters on the Cerbo and both of them working fine again.