This error message occurs when a connection cannot be made with the administration in AFAS. In this case, 'unit' refers to the administration.
Solution:
Go in AFAS to General / Management / Authorization tool.
Go to the tab 'Maintain groups' and select the group used by the App connector.
Go to the tab 'Authorization' and select General / Administrations / Filters.
Then at the bottom choose 'Filter active' for all or the relevant administrations.