Given the case you've unlocked at least up to campaign mission "Bomos" for our repro:
- Start campaign mission "Signal lost"
- Create a user savegame somewhere
- Exit the mission (e.g. by pressing Esc, Alt-F4) or save and exit; in my case I had it properly finished and played until "Bomos"
- Start "Bomos" mission and e.g. listen to the conversation of the three guerillas in order to acquire the scout mission tasks
- Quit mission (note: there is no chance to save in this mission)
- Leave and re-enter the campaign menu
- Mark the "Bomos" mission and click RESUME
- "Signal Lost" mission savegame is loaded