Visma Connect: invalid_tenant on exchanging authorization code for access token

[phoinv]

We have an user of our app registered under Visma Connect which receives an invalid_tenant error message when exchanging an authorization code for an access token.

The HTTP POST to /connect/token is similar to the one mentioned in this topic: https://community.visma.com/t5/Forum-in-Developers-Visma-net/Error-redeeming-code-invalid-tenant-no-... and it is according to the specs. The user is an integration administrator for their company.

What could be the reason for this error message? Any information you need to diagnose the issue?