I don't think anyone has talked about this possible problem with DISM failing:
If your boot / EFI partition is full or running of out of room, DISM will fail.
As to why it happens, that's because Microsoft has genius designers (sarcasm).
Typically that partition (first one) is 100 MB. On updates Windows writes stuff in there, so it can fill up.
To resize it you either have to reformat and reinstall Windows or use a partition tool (Aomei, EaseUS, etc.).
Using a partition tool to resize the boot partition is worth a try, but it's also likely to break everything anyway, which takes you back to reinstalling Windows.
After 5 pages of DISM discussion, a side comment is warranted: I've never found a tool like scannow and/or DISM capable of fixing anything. These tools are not much more than placebos. If they report no errors they make you feel good, but you can never be sure. If they report an error, it's some gobbledygook that's difficult to understand or relate to your observed problem. In other words, the anxiety over DISM is not worth it.
