We are aware of this bug and fixing it is high priority for us. The cause appears to be something interfering with autosave and autosave_1, preventing further current_geoscape saves to be generated so the game goes back to the previous one. Unfortunatelly we still do not know what causes it exactly. It could be an anti-virus, it could be an windows update, it could be literally anything. But we have some ideas for a workaround and will continue looking into it.
We have enough reports about it now so you do not need to report it again. Sorry for the problems.