First, be sure you both have the same command line set in Steam (be sure -debug option is off).
Next be sure you both have identical Game and Launcher versions:
View attachment 647443
Current Game version is 1.10.1, Launcher is 2020.14
If the version or checksum on this screen is different you need to upgrade to the same version of HOI and/or "Validate your Game" in Steam.
If the two are the exact same, it is not your games, but mods that are different.
Next, be sure the mods you have are the exact same mods, and the same versions.
Even a tiny mod change will change the checksum
With the recent patch/update release, many mods are being changed also, so most will need to be updated.
This screen shows what the checksum
including mods is:
View attachment 647460
If the checksum on this screen does not match and the first checksum did match, it must be one of the mods you are using is different.
Hope this helps you.