Since HOI4 is 32-bit, it needs contiguous memory. Windows often limits this.
A common cause of instability in 2021 was the use of incompatible sub-mods. Millennium Dawn has a vibrant sub-modding community, but these often break the checksum. hoi4 millennium dawn crash fix 2021
While Steam HOI4 is usually LAA enabled, the 2021 Beta patch (1.10.8) broke this flag for some users. Since HOI4 is 32-bit, it needs contiguous memory