have you installed fmtx faker? may help on first run, it will "patch" your fmtxd.
Seems to be working. But the only problem is that doesn't want to turn off and giving that message Unable to use FM transmitter and its happening only when using bellow frequency 87.5 Mhz (in my case). The only way i can turn it off is by FMTX widget. Which is ok for me as long as it works and power_level still at 120 without fmBooster.