A symptom of this is the following error message on startup:

Error HRESULT E_FAIL has been returned from a call to a COM component

or

Unable to cast COM object of type 'Microsoft.Office.Interop.Outlook.ApplicationClass' to interface type 'Microsoft.Office.Interop.Outlook._Application'

There is no one definitive answer for this. In most cases it is caused by an incorrect or partially corrupt installation of Microsoft Office. Doing a complete uninstall and reinstall of Office usually fixes it.

If uninstalling did not work, you can also try the Microsoft Office Cleanup Tool to wipe any traces of Office before reinstalling.

If you get this error on startup:

All of the configuration for Outlook on the Desktop is done through the tray icon for the program. That is the white icon with the day of the month on it in the bottom right corner of your screen. Right click on it to access all the configuration options.

Unfortunately, there is no way to fix this.  Microsoft decided to strip this functionality from Windows Vista and above.  However, there is a trick that still works:

Hold the "Windows Key" and press M to minimize all windows.  It will minimize everything except Outlook on the Desktop, essentially achieving the same effect as Show Desktop.  "Windows Key" + Shift + M will restore your minimized windows.

OotD runs on Windows 7 and up, including 8.1 and 10 in both 32-bit and 64-bit variants. If you are still on Windows XP, you can use version 3.1 or lower, which can be found on the Release Notes page.

First check to see if the issue was already report here. If not, please create a new issue and I'll have a look as soon as possible.

This is usually due to an issue with either your PST/OST file or with your Outlook Views. Try the following steps to resolve it:

1. Press your Windows Key + R and then in the run dialog type: Outlook /cleanviews and press enter.
2. If that didn't work, try following the instructions here to repair your OST/PST

At the present time, no. However, I am looking at adding the ability to do this if it's possible.

Unfortunately not. As far as I can tell, Microsoft has not provided any way of hosting the Outlook Today page inside of of the Outlook View Control (which OotD uses). In fact, it was meant to be used the other way around. Outlook Today is meant to contain the outlook view control to customize it.

The easiest way to go back to month view is to right click anywhere inside the OotD window and click "Goto Date" then select "Month View" from the pull down.

Close OotD, then open Microsoft Outlook and set up the calendar the way you would like to see it in OotD and then close Microsoft Outlook. The next time you start OotD it will load with that layout.