I am the original poster and reporter of this issue on the Steam discussion forums. A total of five people on the Steam discussion thread are reporting this issue including myself at the time I am posting this.
I also opened a support ticket with my Lock n Load account but the ticket was closed(!) after one week with no explanation and I was told to come to this forum.
Below is what I put in the now closed LnL ticket:
The game started crashing for me on both my Mac and PC computers two days ago (late June 2020).
I can select a scenario and click Start. The game map with the units briefly appears and then the game crashes and I am returned to the Steam launcher.
This is happening on both my Windows 10 PC and macOS Macbook Pro. Reinstalling the game or running in windowed mode do not fix the problem.
The only possible clue I have found is that I see this missing file message on both my Mac and PC in the app logs (mac version pasted below), which is strange since I have never modified the contents of the game folder since it was installed a long time ago:
Could not find file "/Users/<redacted>/Library/Application Support/Steam/steamapps/common/Tank On Tank Digital - West Front/LinkedUnits.txt".