Thanks!!!!
I followed the advice with the import folder, and it seemed to work again. I scanned a few documents, and they all ended up in Evernote as expected. Then suddenly it stopped working. Pdf's ende up in the desktopfolder, but were not transferred to Evernote anymore. There was a flash of a red pop-up for a fraction of a second that said that something was wrong with my account. I checked the connection to the desktopfolder, seemed to be normal. I tried to drop the pdf's from the desktop folder into Evernote, but all files remained grey as if they have not the correct filetype although some of them were already correctly imported just before.
When I open a file from the desktopfolder with PDF expert and print it directly to Evernote it seems to work (even though the filename is the same except that it ends ...pdf.pdf).
I also checked my account, but it was payed for until next year.
I really appreciate your effort and swift responses.