Did you manually delete records from the database. Not using the mendix action, but directly through the database viewer/manager?
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When deleting objects directly using the database manager, you will run into data corruption.
Since associations are maintained by the Mendix runtime, when deleting an associated record, the record of the association will stay in the database, when deleting data manually. When executing a delete through the Mendix runtime, Mendix will delete association records as well.
Thus in short: never manage the data in the database using the database manager (unless you must and know what you are doing) always use the Mendix runtime. by using microflows or a overview page for an sys admin.
To fix your issue, delete the records in the association table manually, to fix your issue created manually.