VST instrument causes Mixer to be blank and prevents score from playing

I added a VST instrument (XLN Addictive Drums 2) to a MS4 score and everything was okay - the playback worked fine. However when I subsequently try and open the score, I can see the instruments loading one at a time in the mixer but the VST and all remaining instruments appear blank (see attached screenshot). The score also will not play (the blue bar remains at the start of the score). If I close and reopen the mixer, the instruments that previously loaded disappear and it appears totally blank. The only work-around I have found is to delete the instrument, close and reopen MS4 and then add the instrument and VST back in. This is unsatisfactory, however, because all the VST settings are lost.

Has anyone else experienced this problem and come up with a better solution? I'm fairly desperate to find a fix!

As a side note, I also found when entering my drum part that occasionally this one instrument will stop playing at an entirely random point in the score. If I go to the start of the score and try again, the drums stop playing at the same bar. I've found I can work around this by adding a forte dynamic at the start of the problem bar - after doing this everything works fine and I can remove the forte mark.