Hello David,
Thank you for your reply.
I am getting a better understanding of the situation now.
The login I used does actually exist but is not the login of the user, and was not given permission to the Zetadocs site. Once I gave this permission the error disappeared.
The only problem is that the credentials used for login are not the "Office 365 username and password" of this user but of another one.
I am thinking of deleting and recreating the user is Navision to see if it gives me a chance to enter the credentials again. If you know an other way, such as clearing some fields in one particular table, I'll be glad to know of it.
Otherwise I could just leave it as it is. If the only impact is to give an extra sharepoint access to the Zetadocs site, I don't see it as an issue. In that case would it be possible/sensible to use the same Zetadocs login (dummy Office 365 username and password) for all users?