It is well known that iMini has a huge problem with Virtual MIDI (stuck notes etc).
I think I found what might be the cause.
When clicking on MIDI Learn and then on a knob, you can see if there is a mapping already set up. I don't remember setting any mapping before but it appears that iMini has mapped many controls on the last Virtual MIDI port I tried WITHOUT me pressing the MIDI Learn button. So, the explanation might be that iMini gets by mistake on MIDI Learn mode, and that's why Virtual MIDI gets stuck...