When trying to migrate, or running a pre-check on a file from Office 365, you receive the following error message:
The folder name 'FOLDER_NAME' contains one or more special characters (e.g. '#' or '%') that prevent Sharegate from fetching its files. Renaming the folder to remove these characters will allow Sharegate to copy its content.
This issue stems from the Microsoft API and there is no workaround at this time. This only applies specifically to files migrated from an Office 365 source.
You will need to rename the folder to remove the problematic special characters, and run the migration or pre-check again.