Hi,
I think I might have sorted this by unloading all the Structural and HVAC disciplines in all the *.dgnlib files, in particular:
C:\Program Files (x86)\Bentley\AECOsimBuildingDesigner V8i Ss4\AECOsimBuildingDesigner\AECOsimBuildingDesigner.dgnlib. Fingers crossed.