Diagnosis
You have attempted to change the assignment period for a master tenantwith a customer account. This is not permitted.
The assignment period for a master tenant with a customer accountalways begins on the day the partner is assigned (using the functions'Create/assign partner' or 'Change tenant with customer account'). Theassignment period always ends when the master tenant is changed in thecontract. The end-date is then the day before the new assignment.