Different starting months in the contracts for account &1

What causes this issue?

The starting month &V3& is allocated to contract &V2& of contract
account &v1&. A different starting month (&V&) has been entered in the
other contracts of this contract account that are to be invoiced
jointly.

System Response

Invoicing of this contract account was terminated for this invoicing
unit, because a clearly identifiable starting month must be used for
all contracts that are to be invoiced jointly.

How to fix this error?

Maintain the concerned contracts accordingly.

Error message extract from SAP system. Copyright SAP SE.