Ok. This happened after I deleted a user. We were testing the roles assigned to the user in our UAT environment.
1 - We kept assigning and removing roles to the users but nothing was happening, so we decided to delete the user.
2 - We deleted the user, a notification appeared, mentioning that 8 batch jobs assigned to the user were not going to be completed
3 - I tried to import the user but i only got the error message "Your user objectID is null. Please contact your System Administrator for further assistance"
4 - I imported the user using the Admin user and it was imported without any problems
5 - I tried to use the user import functionality using my account but the error message kept displaying for around an hour.
Thats all the info that I can provide. I hope it helps other people who face this same error.