1) Save/Quit ED from XB1 Dashboard and Reboot the game. (Usually fixes the problem and allows you to accept those previous glitched out mission offerings)
2) Try another system. (If option 1 doesn't fix the problem - But 1 seems to 95% of the time)
Yeah I get it quite a bit, sometimes just going into outfitting then back to the bulletin board kinda resets things. Not all the time though. If not just follow GG7 advice.