KeyError: 'delays' for people who also use AnkiDroid

You may receive an error message while reviewing, that looks like the following:

 Traceback (most recent call last): 
  File "C:\cygwin\home\dae\win\build\pyi.win32\anki\outPYZ1.pyz/aqt.webview", line 21, in link 
  File "C:\cygwin\home\dae\win\build\pyi.win32\anki\outPYZ1.pyz/aqt.reviewer", line 318, in _linkHandler 
  File "C:\cygwin\home\dae\win\build\pyi.win32\anki\outPYZ1.pyz/aqt.reviewer", line 251, in _showAnswer 
  File "C:\cygwin\home\dae\win\build\pyi.win32\anki\outPYZ1.pyz/aqt.reviewer", line 617, in _showEaseButtons 
  File "C:\cygwin\home\dae\win\build\pyi.win32\anki\outPYZ1.pyz/aqt.reviewer", line 654, in _answerButtons 
  File "C:\cygwin\home\dae\win\build\pyi.win32\anki\outPYZ1.pyz/aqt.reviewer", line 637, in _defaultEase 
  File "C:\cygwin\home\dae\win\build\pyi.win32\anki\outPYZ1.pyz/anki.sched", line 135, in answerButtons 
  File "C:\cygwin\home\dae\win\build\pyi.win32\anki\outPYZ1.pyz/anki.sched", line 595, in _lrnConf 
  File "C:\cygwin\home\dae\win\build\pyi.win32\anki\outPYZ1.pyz/anki.sched", line 1099, in _lapseConf 
  KeyError: 'delays'

This is caused by a bug in AnkiDroid 2.4, where it doesn't create filtered decks correctly. Please remove any custom study & filtered decks that you created with it, and recreate them on the computer version to fix the problem. Please note that rebuilding is not sufficient; you need to remove the old copy first.