Outlook 2016 -- The custom form cannot be opened. Outlook will use an Outlook form instead. The form required to view this message cannot be displayed. Contact your administrator.

Hi yleclerc- [O365],

Since you hae tried to clear cache and it doesn't work for long. It is suggested to check if Outlook item uses correct Form.

1. In navigation pane, locate the folder where you want to create a new item.
2. Then right click it and choose “Properties” from the context menu.
3. In the popup dialog box, under “General” tab, you can see its from in “When posting to this folder, use: ”
4. You should know that the form for emails is IPM.Post, for calendar items is IPM.Appointment.

If it doesn't work, you can try to manually clear the Forms Cache:

1. Go to “C:\Users\%username%\AppData\Local\Microsoft\FORMS”
2. Then delete all items in the folder. (Please back up it before you delete it)
3. Check if the issue persists. 

If it doesn't help, could you please tell me the version of Outlook and the detailed reproduce steps. 

Regards,

Carlos

As is stated before, I have tried most of the suggested methods of repair.  

I have checked the "IPM. " types but how can I select to type of post when my calendar is automatically sending this type of message to my Sent box.

Here is a sample message:

<?xml version="1.0"?>
<CalendarNotificationContent xmlns:xsd="**Removed**" xmlns:xsi="**Removed**" Version="1.0">
  <CalNotifType>Reminder</CalNotifType>
  <CalNotifTypeDesc>Appointment Reminder</CalNotifTypeDesc>
  <CalEvent>
    <DayOfWeekOfStartTime>Monday</DayOfWeekOfStartTime>
    <DateOfStartTime>3-26-2018</DateOfStartTime>
    <TimeOfStartTime>4:30 PM</TimeOfStartTime>
    <DayOfWeekOfEndTime>Monday</DayOfWeekOfEndTime>
    <DateOfEndTime>3-26-2018</DateOfEndTime>
    <TimeOfEndTime>5:00 PM</TimeOfEndTime>
    <Subject>GB2-4</Subject>
    <Location>**Removed**</Location>
  </CalEvent>
</CalendarNotificationContent>

What next?

----Personal Information Removed by Moderator----

Hi yleclerc- [O365],

As far as I know, Outlook won't send this kind of message automatically. May I know if you have installed third party add-ins? Please try safe mode and check if the issue persists.

Besides, please make sure the "IPM" type of your calendar is IPM.Appointment, and the "IPM" type of sent items folder is IMP.Post. 

If the issue persists, please back up your data and have a repair. And please let me know if it works. 

Regards,

Carlos

Hi yleclerc- [O365],

If you get any update, please share at your convenience.

Thanks,

Carlos