I’ve found the same thing with the PC3 editor I’m building. It doesn’t seem to be multiclient. I thought it was the PC3’s MIDI interface, but it might be Ctrlr
In ableton, it’s pretty easy to work around, just turn the MIDI device off in Ableton and set it with the Ctrlr VST. You can then reroute Ableton tracks to send to the track with the Ctrlr device on it, and it works as normal.
Another option might be one of the virtual MIDI drivers (like MIDI Yoke or CopperLan), where you can use the virtual MIDI cable as a sort of sacrificial MIDI port and just forward it to your normal MIDI interface.
The Puppeteer
http://godlike.com.au