Hi Lauens,
please check your changes on the code if you have changed the cardinality from an 1_1 association to an 1_n association. If you have already code on the mendix cloud which is using the 1_n association, this could bring up this error. To solve that problem, you need to migrate the data to the new structure, by using a script for example.
If you run your database from scratch, the error should not persist.
Thanks for the reply! Funny thing is that I do not have these kind of problems when I deploy it to the Sap Cloud Foundry Environment. I started from scratch at both environments. Strange.
Do you also have the problem on the mendix cloud when you start the appolication from scratch?