When I tried to update MODO DRUM from 1.1.0 to 1.1.1 via Product Manager, the MD installer showed a popup saying "You already have a newer version installed".
So I "canceled" the installer, which made PM launch the installer again, which showed the popup again. After 4 unseccussful attempts to stop the update, I just clicked "Install anyway". This worked fine.
Probably the method that the MODO DRUM installer uses to check the already installed version has a bug.
So the simple workaround is ignore the error message and install MODO DRUM 1.1.1 anyway.