MIDI parts linked/unseparable

I’m on Cubase 6.5.1 (64 bit) on Mac OS X 10.6.8

Whenever I create a new midi part it automatically links to a previously created midi part that’s completely unrelated (different tracks, output, instrument etc). When I now double-click the midi part, all midi parts are linked in the key editor. I’ve been a Cubase user for more than 10 years but I’m clueless to what’s going on here.

Any input would be greatly appreciated!

A different MIDI problem, but perhaps the same general solution could be found here?
Be careful, though, save your prefs first.