Importing assets: can't get users linked to assets

Importing assets: can't get users linked to assets

I'm working on importing a number of assets into our Service Desk. I'm having problems though. When I was testing it out, everything worked perfectly. The users whose usernames I'd put in the CSV file were assigned correctly without problems. Now when I import something I get an error message. I know the domain name is correct as it hasn't changed in the last 10 years. I also know the usernames are correct. With that in mind I don't understand the error message. It tells me to check if I've typed everything correctly but I know I have.

Is there anybody who could help me with this?

Here is the message I'm getting:
this?


              New to ADManager Plus?

                New to ADSelfService Plus?