Solved
"Mission Select" options disappeared
Earlier today I finished the GDI campaign and was browsing through the menus, on the "Mission Select" screen I saw that I was able to replay all my previous missions as well as the alternative A/B/C ...
This might be due to keeping too many savegames; a side effect of the handy quick save feature and the fact savegames can't be selected for overwrite, combined with a full screenshot being saved per savegame as "thumbnail".
Try deleting a bunch of your savegames and see if that remedies the issue.
