I got the advise to reset some dat-files to solve my issue. Where can I find these and what do they do?
The most common dat-files that can be reset for troubleshooting are outcmd.dat and extend.dat. To reset them you can browse to their location and rename them to .old when Outlook is closed.
Keeps toolbar customizations. Resetting this will restore the Toolbar to default and recreates the icons that some add-ins might have installed.
Keeps a registry cache of installed Exchange Client Extensions add-ins and their status. Resetting this will force Outlook to retrieve the settings from the registry again and reset their status.
In Windows XP you can find the files in the following locations;
C:\Documents and Settings\%username%\Application Data\Microsoft\Outlook\
C:\Documents and Settings\%username%\Local Settings\Application Data\Microsoft\Outlook\
Windows Vista and Windows 7
In Windows Vista and Windows 7 you can find the files in the following locations;
Note 1: To easily locate the folders for you, you can also use OutlookTools (free!)
Note 2: For an overview of the locations of other Outlook files and settings see this guide.
Note 3: Outlook 2010 no longer uses the the extend.dat and outcmd.dat file.