I have had this error before on X1 and I posted about it in the forum. I was told that it shows up sometimes if you use simple instrument tracks rather than the full on tracks. I have since changed to only using full on tracks and will not be using simple instrument tracks anymore.
If you are using simple instrument tracks you should try splitting them into individual, full on tracks (right click, seperate track).
If this bug is still in X2 it needs to be addressed now.