S412: Fax: PDFs are not created

Hi all,
today i noticed while accessing my S412 via ssh that many faxes (of which I thought they never arrived) are only saved locally as tiff. They are not converted to pdf and not forwarded by mail like it is configured in the trunk preferences. The System log shows, that the pdf was created, but it doesn't exist in the fax Folder on the S412 and so it never reached me with via fax-to-mail.

The interesting Point is that this bug appears depending on the Sender. Faxes from the same three or four senders are forwarded regularly as pdf. But other faxes are not converted to pdf and stay as .tiff on the S412.

Any Ideas anyone?