Lay it on me. Please include any relevant information like your operating system/hardware specs/if you've recently updated/are experiencing other issues that may be related :)
Unless specifically noted game updates should not break save files. However, a game version update will make your save out of date.
Editing your Save.bum file to change the SaveVersion line (the second line of the save file) to the currently installed version of Steam Marines 2 should allow the game to load the file correctly, for example SaveVersion v0.7.0a to SaveVersion v0.7.1a
The Save.bum file, as well as other config files, can be located at:
Windows: "\Users\<User>\AppData\Roaming\Worthless Bums\Steam Marines 2\"
Linux: "~/home/<User>/.config/Worthless Bums/Steam Marines 2/"
The game log file can be located at:
Windows: "\Users\<User>\AppData\LocalLow\Worthless Bums\Steam Marines 2\output_log.txt"
"~/.config/unity3d/Worthless Bums/Steam Marines 2/output_log.tx"
You can also reach me via email at firstname.lastname@example.org