I have the exact same issue. After this happens, when trying to verify the game files, Steam reports that one file could not be verified, and downloads a 3.7mb file to "fix this" but it doesn't seem to help as the same issue occurs anyway. I've done multiple tests on what causes it and it seems to be something while exiting the game that glitches the game files. Just starting the game and then pressing exit on the main menu causes this for me. I've turned off AV realtime protection, firewall's HIPS and firewall itself, made virtual memory size bigger, restarted the computer numerous times, updated drivers and computer. To no avail unfortunately. Verifying the files a second time in a row verifies that all files were validated succesfully.
Only solution I found was to uninstall the game and install it again. I was then again able to play my saves, but once again, after I was done with play session and exited the game, next time I started the game the same issue with not being able to load the saved games occurred. Not going to uninstall/install the game after every session.
Interestingly the first time I loaded a saved game after reinstalling, the music in the loading screen was quite loud, usually when loading the sound volume for the music goes really quiet. The music volume in the loading screen was again on low volume after the play session was over and I started the game again. I thought I also noticed some new music which I had never heard before in the play session after reinstall, so maybe it's something sound related. Or something had gone wrong much earlier in my install (original install was from 2015 which had been updated to newer versions over the years).
Problem Event Name: APPCRASH
Application Name: LegendaryHeroes.exe
Application Version: 18.104.22.168
Application Timestamp: 5cdd7813
Fault Module Name: LegendaryHeroes.exe
Fault Module Version: 22.214.171.124
Fault Module Timestamp: 5cdd7813
Exception Code: c0000005
Exception Offset: 000fc837
OS Version: 6.3.9600.2.0.0.768.101
Locale ID: 1033
Additional Information 1: 5861
Additional Information 2: 5861822e1919d7c014bbb064c64908b2
Additional Information 3: 1318
Additional Information 4: 13181ae637ac2592fac0ca2dd662cdcc
/// UPDATE: Found a workaround that kinda works for me. Verify files in Steam, start the game, and create a new game. When the new game loads, go back to main menu and now load the save game. When trying to load directly from the initial main menu the game crashes, but from within an ongoing game it works fine.