Special characters were changed. Check project editing masks and change

What causes this issue?

You changed the special characters. This can affect existing project
definitions, WBS elements and their standard structures.
If a new character is used, make sure that this character is not
already saved with an external number on the database (use the
appropriate object browser for this).
If the character does have an external number, you might not be able to
process the objects.
If you delete characters, check the editing masks. You should not
delete the characters if they are used in masks.
Sometimes in this case, the objects can no longer be processed.

System Response

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

How to fix this error?

Error message extract from SAP system. Copyright SAP SE.