If a custom campaign has more than 5 chapter slots, you will not be able to save during the campaign. Try it yourself:
Download this campaign, which has 5 chapters in total, and try to save the game, either using F6 for quick save or the normal way using the save function in the game menu. It will work fine.
https://www.eredalis.de/warcraft-3-reforged/heroes/prologue/
Then just download this campaign with 10 chapters in total and try to save (F6 or regular) in the first mission. The game will immediately CRASH! This problem has existed since the re-release of the custom campaign mode.
https://www.eredalis.de/warcraft-3-reforged/heroes/human/
More information: My system is Win10 Home. When trying to navigate back to the Warcraft III start screen from the custom campaign start menu, the game also crashes…
I have received several reports from other players who are also experiencing these problems.
/Edit: And I bet my boots that this reported problem here… 
…has also to do with it. 