[2020] Can I host a MP campaign from a save file from another host?

Hi! Yes, you should be able to transfer a save file like that. However, besides the save file itself, there's also a file called "[the_name_of_the_savefile]_CharacterData.xml" in the same folder as the save file, which is used to save the player characters (skills, inventories, etc). This file will also need to be shared if you want to keep playing with the same characters. With both files, you shouldn't have trouble resuming the campaign!