Changed milestone data not available

What causes this issue?

The milestone has been deleted, possibly by another user in the shared
mode.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

<ZH>Note</>
The shared mode is available if you have deactivated the default
Business Add-In (BAdI) implementation <NP>BADI_IMPL_PS_SHARED_MODE</>.
For more information, see Customizing for <LS>Project System</> under
<LS>Handling Large Projects</> -> <LS>Business Add-Ins</> -> <LS>Make
Settings for Project Editor</>.

System Response

This message is displayed when you try to save milestone data that has
been simultaneously edited by another user in the shared mode. In this
case, this user has deleted the milestone in the shared mode.
The system displays a message that the changed milestone data is not
found.

How to fix this error?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.