Hi Dennis,
The following steps are for your reference:
1.Check if the default email account in Outlook is the same with the user which is used to log into SharePoint Online.
If not, the problem may occur. Just see your another Office 365 P1 account, only one user, there is no problem.
2.Choose an affected user, remove the connected SharePoint contact list from Outlook by clicking File->Info->Account Settings->SharePoint lists.
•If the issue persists, it indicates the issue is related to the credential for Outlook. You can go to Control Panel->Credential Manager->click the credentials to remove them. Then, restart Outlook.
•If not, reconnect the SharePoint contact list to Outlook:
Note: Log out SharePoint Online first , and then log in again with both the Remember me and Keep me signed in checked.
Thanks,
Cherry Wang