To reproduce: 
- Set the text of the RadMaskedEditBox.
- Set The text again and you will notice that the second time the text is not set.

Workaround:
radMaskedEditBox.Value = null;
radMaskedEditBox.Value = _dateValue.ToString("dd.MM.yyyy");

Resolution: 
Set the InsertKeyMode to Insert/Overwrite of the StandartCharacterMaskEditBoxProvider to turn off/on text insertion. Here is the code snippet: 
StandartCharacterMaskEditBoxProvider charProvider = ((StandartMaskTextBoxProvider)this.radMaskedEditBox1.MaskedEditBoxElement.Provider).TryGetStandardProvider();
charProvider.InsertKeyMode = InsertKeyMode.Overwrite;