IoT core: Bulk registration failure

Hello
I am trying to figure out the possibilities for bulk registration of IoT devices which has led me to these guides:
https://aws.amazon.com/blogs/iot/deploy-fleets-easily-with-aws-iot-device-management-services/
https://iot-device-management.workshop.aws/en/provisioning-options/bulk-provisioning.html
I have arrived at the step where I need to run my "aws iot start-thing-registration-task" command, but I get the error: "'inputFileKey' failed to satisfy constraint: Member must satisfy regular expression pattern: [a-zA-Z0-9!_.*'()-/]+" At first I tried following the guide to remove the begin and end line of the csr along with making it newline delimited, but I got the same error. I then tried using the "mk-bulk.sh" from the workshop and used that json file which gave the same error along with an error saying that the file is above the 1024bytes limit of the AWS CLI function.
The guides are from 2018, so things might have changed, but they were the most recent I could find.
Any help is much appreciated and let me know if you want me to provide my one-line json.

Best regards
Martin Pedersen