After re-installation of Foxit, everything is fine again. In Outlook the Preview Handler fails too. However, after a couple of days the PDF Preview Handler does not work anymore and gives the error message 'Due to an error in the PDF Preview Handler, this file cannot be previewed.' (Translated from German, in English the message might be slightly different).
12.4 the PDF Preview Handler works well in the Windows Explorer Preview pane and in Outlook.
Directly after the installation of Foxit PDF Reader Ver.