Dear LN!
I encounter some problems with TrackIr in combination with the Mig-21bis. It seems that TrackIr only works when the Mig 21 is the first module i fly with after starting DCS.
When flying with another aircraft and switching to the Mig 21 causes the TrackIr to stop working with the Mig21. This doesn't mean that the TrackIr software crashes, it is still active. The pilot's view simply does not respond to my head's movements. Other modules are not affected.
I also realized that, since 1.5.4, in combination with every module, in the upper right corner a notification, regarding the headtracker appears after the mission start, except for the Mig21 (and for the Gazelle by polychop as well, btw). And it also seems to me, that this issue appeared only since the 1.5.4 update (IIRC..not sure)
Maybe you are already aware of this bug but according to cobra's words: better safe than sorry and i haven't found anything else neither in the forums nor in the bugtracker.
safe landings