Ok, I was seraching/debugging in the meanwhile on my own.
I found out that executing the following, block sheets for other users (see red rectange on image below).
Dim sheets As ViewDraw.IVdObjs
'# this line below locks in a concurrent design
'# all other pages on other clients which are
'# not active open
sheets = vdApp.QueryPages(jobSchematicName)
To release this object, at least the following line is necessary.
or to make it a little more professional:
'# For programs with multiple COM objects declared, repeat the following two
'# lines of code for each COM object in reverse order of creation.
Catch ex As Exception
Me.UpdateRtb("Internal Error: " & ex.ToString, Color.Red)
obj = Nothing
'# After all of the COM objects have been released
'# and set to null, do the following:
'# Start .NET CLR Garbage Collection
'# Wait for Garbage Collection to finish
This removes the "Locked by xxxx on Computer yyyy" Message on top.