Hi Rene,

If Outlook worked correctly the last time and nothing has changed about your account configuration, then checking if things work correctly in Safe Mode is a good first step.
Starting Outlook in Safe Mode will prevent some or all add-ins from being loaded, prevents several settings files from being loaded and turns off the Reading Pane.
To start Outlook in Safe Mode you’ll need to start Outlook with the “safe” switch;

• Windows Vista and Windows 7
  Start-> type; outlook.exe /safe
• Windows 8
  Start Screen-> type; outlook.exe /safe
  (just start typing, a search box will automatically become visible)
• Windows 10
  Start-> type; outlook.exe /safe
  (just start typing, a search box will automatically become visible or type in the command in the Cortana Search box)

If it works in Outlook Safe Mode, the issue is most likely caused by an add-in, a corrupted settings file or the first message that is being displayed when Outlook is started.

Disable add-ins

If Outlook works in Outlook Safe Mode but resetting your dat-files did not help, then the issue is most likely caused by an add-in. Often, but not always, this is accompanied by an error similar to:

“The add-in <reference to add-in> could not be installed or loaded. This problem may be resolved by using Detect and Repair on the Help menu. Unable to load <reference to add-in dll>.“

The instructions given in the error will seldom actually fix this issue as the issue is more likely to be caused by the add-in itself, rather then by an Outlook installation issue.

Start Outlook in Safe Mode again and disable all your add-ins. After this, you can start Outlook normally again and see if it works now. If so, you can enable them one by one until Outlook fails to load again. The add-in you last enabled is most likely the cause of your issue. Uninstall or reinstall this add-in or check with the vendor for updates or contact them for further troubleshooting their add-in.
You can enable/disable Outlook add-ins in the following locations:

1. File-> section Options-> section Add-Ins
2. At the “Manage” dropdown list at the bottom select which type of add-ins you would like to enable/disable
3. Press “Go…” and make the changes
4. For some add-ins you’ll need to restart Outlook for changes to take effect.

I hope this helps in fixing the Outlook problem for you.

Regards,
Lokesh