I managed to reproduce the described issue and I have logged it in our Feedback portal. You could easily track its status on the following link:
In the explained scenario the RecurrenceDialog Ok Button shouldn't be disabled if the RecurrencePatern Frequency is Monthly or Yearly. What I can suggest you now in order to workaround the issue would be to modify the EditRecurrenceDialog ControlTemplate in order to replace the command of the Ok Button to one in the ViewModel. This way you will be able to confirm the changes if the duration is bigger that 7 days and the RecurrencePatern Frequency is set to Monthly or Yearly and if not you could show error message. For your convenience I have prepared and attached a sample project which demonstrates the exact approach (note that project uses Implicit Styles
Hope this helps. I have also updated your Telerik points for the involvement.
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