So this is a bug that has plagued this game for years now, the infamous broken reward bug. Basically it's when the game fails to load the rewards after you win or lose a battle except now it also kicks you out of the mission, robbing you of your fuel in the process.
While we wait for this kink to be ironed out I have found a simple fix. If you see that you're getting no rewards from a battle (experience and gold at 0) whatever you do, do NOT hit continue as this will lead to the game kicking you out. What you want to do instead is refresh the page, this will boot you back to the autosave before that battle (I know, it kinda sucks having to do it over again) so you can try again.
This glitch only seems to happen when the client is under strain or is running slow which from the past 2 days has been in the late morning and early afternoon by my time. Why I think it's kicking players out is because the game is trying to load something it can not process properly or a value of something that leads to an error although this is pure speculation on my part.