.cookie_ is generated whenever Armory kicks off ArmoryDB. I don't think it'll cause any issues. To be safe, if Armory and ArmoryDB aren't running, delete the cookie and try again. If it still doesn't work, I don't know what's happening. I finally broke down and bought an RPi3 but it'll be awhile before I get around to playing with cross-compiling and running Armory on it. Don't wait on me.