I received an email from Nintex below, went to the site and logged in, after which I had an issue.
As I understand it, you need to follow the instructions (on issue scrin):
User account 'firstname.lastname@example.org' from identity provider 'live.com' does not exist in tenant 'K2 Trial' and cannot access the application 'bd5c2c8b-7c22-47a5-8fc5-bd3f76a315d5'(Azure Active Directory for K2) in that tenant. The account needs to be added as an external user in the tenant first.
Please help me solve the problem, if not, please give me a contact, someone who can.