This a a known OFP bug when save files get too big.
One way I have used to get around this problem is:
1. Before loading OFP, find the mission .fps file and either move it elsewhere or make a copy elsewhere. I usually take the weapons.cfg also.
These files can usually be found in:
OperationFlashpoint\Users\Username\Saved\missions\missionname folder
2. Start OFP and start the mission from the beginning. Get killed, then Alt-Tab back to windows and replace the files you copied back into place again.
3. Go back to OFP and start again from your save position.
Having said all that..........this is for SP missions and I don't know if this sort of procedure will work for campaigns or if they use the same type of files to save your position.
Have a look and see and hopefully this will work as well for campaign saves too.
Planck