Make sure Headtracker.dll is in the bin and bin-mt directories. My first few attempts at updating to 2.9.6.57650 failed and it deleted a bunch of files. I had to do a cleanup and repair but Tobii still wasn't working even after removing and reinstalling all of the Tobii software. I had to manually copy the Headtracker.dll file into bin and bin-mt to get it working again. Something I'm assuming the Tobii software did itself when I first started using it months ago.
Check out this page for more information.
https://help.tobii.com/hc/en-us/articles/18493132485137-Experiencing-Issues-in-DCS-World