I can't reproduce this, @CyrilleB79 can you paste the log message?. It looks like I must have misunderstood how to trigger the initial category reporting. I can report categories on an email, but not on a calendar event.

Here is the log:

IO - inputCore.InputManager.executeGesture (11:41:41.864) - winInputHook (16236):
Input: kb(desktop):tab
ERROR - eventHandler.executeEvent (11:41:41.974) - MainThread (20248):
error executing event: gainFocus on <appModules.outlook.CalendarView object at 0x12A77970> with extra args of {}
Traceback (most recent call last):
  File "eventHandler.py", line 246, in executeEvent
    _EventExecuter(eventName,obj,kwargs)
  File "eventHandler.py", line 96, in __init__
    self.next()
  File "eventHandler.py", line 105, in next
    return func(*args, **self.kwargs)
  File "NVDAObjects\__init__.py", line 1137, in event_gainFocus
    self.reportFocus()
  File "appModules\outlook.py", line 387, in reportFocus
    categoriesText = self._generateCategoriesText(p)
  File "appModules\outlook.py", line 361, in _generateCategoriesText
    return categoriesText.format(categories)
KeyError: 'categories'

Note that reporting categories on message was already supported in NVDA 2020.4. The new feature added in #11598 with the use of ngettext was adding category reporting on appointments. You can add a category to an appointment by context menu on the appointment, the same way as you do for messages.
Anyway, that explains why your manual tests in #12432 were OK.

The code here is fine, and it is obviously a bug. But given that I didn't manually test this effectively, it would be great to have proper steps on one of these PRs or issues. Currently none do.

I still can't get reproduce the error. I'm using Office 365.

• Press ctrl+tab until the "navigation bar grouping" is announced
• Use left / right arrows to find the calendar view button and press enter
• Down arrow until I find an appointment (which I know has a category applied)
• Press NVDA+tab
  Speech from speech viewer:

1:30 PM to 2:00 PM
Has appointment 2:00 PM to 2:30 PM
, , 2:00pm to 2:30pm Thursday, 20 May 2021, 1 event, 1 Busy  cell  Calendar Day View  focused  selected  Calendar Day View

@feerrenrut here are the requested clarifications.

Here is the STR that lead to the error that I have pasted above:

• Open Outlook
• Press control+2 to go to calendar view
• Tab or shift+tab to an appointment in the current week that has an associated category (or more than one)
  I am using Outlook 2016 and the calendar view is 'working week'.

If you do not have an appointment with a category, you can add one the following way:

• Open Outlook
• Press control+2 to go to calendar view (working week for me)
• Tab to an appointment
• Press context menu key
• Select "categorize" item
• Select a category in the sub-menu

Note that with the STR you indicate, I can see 2 differences:

• control+tab does not get me to the navigation bar group; i have to press shift+F6. Would it be a difference between 2016 and 365?
• more important, if I use arrow keys in calendar instead of tab/shift+tab, the appointments are reported but not selected and the corresponding category is not reported, neither before this PR, nor after.

I hope that this clarifies things. Let me know if something is still unclear.

Yes, that helps. I had to tab to the appointment, then reading categories works as expected.

I have added this to the PR description.

@CyrilleB79 This usage will be removed for now, we believe it is causing trouble for the po file checker in the translation system. We can revisit this after the release of 2021.1

I understand, this is not ideal. This matches the behavior of the of many similar plural strings in NVDA. We will look at re-introducing this for the next release.