WBS dates were updated from activities. (See Extras -> Act-WBS)

What causes this issue?

The dates for the WBS elements are determined through the various
business transactions, for example from confirmations or the creation
of activities in the network.
The date changes which could arise this way are not automatically
written to the WBS element.
If you call up time scheduling, the dates in the WBS will always be re-
calculated.

Error message extract from SAP system. Copyright SAP SE.