@yeke_yoke said in Oneplus One installer hangs on "flashing firmware":

Flash system partition using fastboot" the installer hangs on the "flashing firmware" stage.

I have the same problem. I haven't tried disabling the "fastboot" option yet.
For the issue you're having, could it be something similar to the "Update Cyanogen Recovery" feature in the developer settings in CyanogenMod? There might be something similar in LineageOS that could cause issues.