Message
Relation with number 12345678 is not of type customer.
(Translation: Relatie met nummer 12345678 is niet van type klant.)
Cause
The contact in ExactOnline probably already exists and is not a customer, but probably a prospect, supplier or creditor.
Solution
In the source package, change the customer number. Then make a change in the source package, for example by adding a note. The synchronisation package will pick up this invoice update and will sync again, with a changed customer number.
Would you like to perform multiple updates at once? Then edit the package and choose to run the export again. After saving the package, the modified item will be processed again, check this explanation).
You can also see this message yourself in Combidesk. To do this, log in with your Combidesk account and then navigate on your dashboard over your link.
At 'History', click through to the notifications.