3CX Fax server fails

[d.blum]

Hi,
Since a few days we get no fax per mail.
I looked in the activity log, where it says "cause=200 ;text="User hangup"
Fax to an old ATA Device is still working.
In the Systemservicelog i found following error message:

Processing C:\ProgramData\3CX\Instance1\Data\Fax\Faxes\ToSend\fax_1511862697_no1.xml
2017/11/28 10:54:18.352|2092|0013|Info|Fax C:\ProgramData\3CX\Instance1\Data\Fax\Faxes\ToSend\fax_1511862697_no1.tif will be converted to C:\ProgramData\3CX\Instance1\Data\Fax\Faxes\ToSend\fax_1511862697_no1.pdf
2017/11/28 10:54:18.402|2092|0013|Erro|Can't open input file C:\ProgramData\3CX\Instance1\Data\Fax\Faxes\ToSend\fax_1511862697_no1.tif for reading
2017/11/28 10:54:18.402|2092|0013|Erro|Cannot convert tif to pdf
2017/11/28 10:54:18.402|2092|0013|Erro|Not processed files will be moved to 'Failed' folder.

In the folder the xml file is available, the .tif file is not in the folder and the .pdf file is a 0byte file.
Any ideas how to solve this?
The permissions on the folder are correct, the System Service of Windows has full access to the folder and files.
THe 3CX Services are running with locale System Account.

Thanks in advance