“FatalError: The given key was not present in the dictionary”
This error can happen in the import process when we try to export and import a site to a different server.
You have set custom security in a list or library.
UPDATE: This can also be caused by having a Workflow set to this list or library.
set the list or library to inherit permissions from parent and try again the process of export and import.
UPDATE: Another solution is to export without including the user security !!!