Can confirm that the cause of the missions inaccessible bug is exceeding the total amount of EXP required for max level. If you exceed it, it is possible though not guaranteed that you can set your exp to a lower value and resume the game.
The game does not fuss about a character that is substantially overleveled, although I strongly recommend sticking to level 17 (even if it means going in and reducing XP to stay there) in the early game. You can't spend points outside of the four main stats until you progress farther, and 17 has 16 upgrade points to evolve them all for a more rad (and still totally enjoyable, set it to hard for fun times with slightly more challenge, haha). I did all collectables, blacknet, and golded available Radnet and never had to back my EXP down, but far warning not to out level your stats so that you can level up later and spend points! Once you max your level, that's it, and it locks you out of six ugrades because you can't level up to spend the points, even by going back in EXP. So, for the whole starting sandbox in the region (Yellow Zone, that's no spoiler), don't max your level. 17 is perfect.
Once you progress (well, very shortly after you progress) Spoiler:
past the Yellow Zone and into the Green zone
, reasonably early on, you can safely cheat to level 22 to max out all available abilities. One remains locked so don't let yourself hit 23 until it's unlocked.