Thanks Razazel.

I managed to avoid this bug with your help. Below are the steps I took:

1. As 'Resuming' led to the bug described above, I reverted to the previous 'Gori' save point - created after completing the previous mission.

2. I tried to go on a patrol using the quadbike to see if Arma 3 would let me. I had the opportunity to select a location to start the patrol from. However, the first time round, I was sent back to Arma 3's main menu after the loading was complete.

3. I restarted steps 1 and 2, picking up a different patrol start location. This time, the patrol kicked in without problem.

4. I returned to base, which automatically created another save point.

5. I tried launching the Breaking Even mission again... with success!

I guess I'm happy for this issue to be closed.

Thanks again for your help.