@tompsweeney No, I still have the problem.

@olivierlambert thank you for your response.
Here is the /var/log/xensource.log where you can find the error: xensource.log.txt

Can anyone help me figure out what the problem is?
Here is the /var/log/daemon.log: daemon.log.txt

If you need other logs just let me know and I'll attach them too.

@olivierlambert
Thank you for your answer.
If I list PCIs I get the following outpu:

...
04:02.0 Multimedia controller: Philips Semiconductors SAA7146 (rev 01)
...

Is it right, that I have to use '0000:04:02.0' to attach the device to my VM?
What is the meaning of the first '0000:'?
Have you any idea what the meaning/reason of the error can be?

Cannot_add(0000:04:02.0, Xenctrlext.Unix_error(35, "3: No such process"))

Does anyone have any idea why it is not working?

My machine:
Server: DELL T3620 with E3-1245v6 and 16 GB ECC RAM
OS: XCP-ng 8.2

I could not forward a PCI DVB-C card to a VM.
All needed steps for it were already done (see https://xcp-ng.org/docs/compute.html#pci-passthrough

1. I need to forward the SATA Controller (00:17.0) and the DVB-C card (04:02.0)

]# lspci
00:00.0 Host bridge: Intel Corporation Xeon E3-1200 v6/7th Gen Core Processor Host Bridge/DRAM Registers (rev 05)
00:01.0 PCI bridge: Intel Corporation 6th-10th Gen Core Processor PCIe Controller (x16) (rev 05)
00:02.0 VGA compatible controller: Intel Corporation HD Graphics P630 (rev 04)
00:14.0 USB controller: Intel Corporation 100 Series/C230 Series Chipset Family USB 3.0 xHCI Controller (rev 31)
00:14.2 Signal processing controller: Intel Corporation 100 Series/C230 Series Chipset Family Thermal Subsystem (rev 31)
00:16.0 Communication controller: Intel Corporation 100 Series/C230 Series Chipset Family MEI Controller #1 (rev 31)
00:16.3 Serial controller: Intel Corporation 100 Series/C230 Series Chipset Family KT Redirection (rev 31)
00:17.0 SATA controller: Intel Corporation Q170/Q150/B150/H170/H110/Z170/CM236 Chipset SATA Controller [AHCI Mode] (rev 31)
00:1b.0 PCI bridge: Intel Corporation 100 Series/C230 Series Chipset Family PCI Express Root Port #17 (rev f1)
00:1c.0 PCI bridge: Intel Corporation 100 Series/C230 Series Chipset Family PCI Express Root Port #1 (rev f1)
00:1f.0 ISA bridge: Intel Corporation C236 Chipset LPC/eSPI Controller (rev 31)
00:1f.2 Memory controller: Intel Corporation 100 Series/C230 Series Chipset Family Power Management Controller (rev 31)
00:1f.4 SMBus: Intel Corporation 100 Series/C230 Series Chipset Family SMBus (rev 31)
00:1f.6 Ethernet controller: Intel Corporation Ethernet Connection (2) I219-LM (rev 31)
02:00.0 Non-Volatile memory controller: Sandisk Corp WD Black SN750 / PC SN730 NVMe SSD
03:00.0 PCI bridge: Texas Instruments XIO2001 PCI Express-to-PCI Bridge
04:02.0 Multimedia controller: Philips Semiconductors SAA7146 (rev 01)

2. tell XCP-ng not to use this device ID for Dom0

]# /opt/xensource/libexec/xen-cmdline --set-dom0 "xen-pciback.hide=(00:17.0)(04:02.0)"
]# reboot

3. list assignable devices

]# xl pci-assignable-list
0000:00:17.0
0000:04:02.0

4. add DVB-C card to my VDR VM

]# xe vm-param-set other-config:pci=0/0000:04:02.0 uuid=799e9021-e4e1-8824-cfaa-7c53c04b216b

5. starting VM

]# xe vm-start uuid=799e9021-e4e1-8824-cfaa-7c53c04b216b
The server failed to handle your request, due to an internal error. The given message may give details useful for debugging the problem.
message: xenopsd internal error: Cannot_add(0000:04:02.0, Xenctrlext.Unix_error(35, "3: No such process"))

It is very strange. I don't have problems to forward the SATA controller to my TrueNAS VM. It works fine. But with the DVB-C card it doesn't work :'-(.
For analysing here is the output of lshw:

        *-pci:2
             description: PCI bridge
             product: 100 Series/C230 Series Chipset Family PCI Express Root Port #1
             vendor: Intel Corporation
             physical id: 1c
             bus info: pci@0000:00:1c.0
             version: f1
             width: 32 bits
             clock: 33MHz
             capabilities: pci pciexpress msi pm normal_decode bus_master cap_list
             configuration: driver=pcieport
             resources: irq:180 memory:ef000000-ef0fffff
           *-pci
                description: PCI bridge
                product: XIO2001 PCI Express-to-PCI Bridge
                vendor: Texas Instruments
                physical id: 0
                bus info: pci@0000:03:00.0
                version: 00
                width: 32 bits
                clock: 33MHz
                capabilities: pci pm msi pciexpress normal_decode bus_master cap_list
                resources: memory:ef000000-ef0fffff
              *-multimedia
                   description: Multimedia controller
                   product: SAA7146
                   vendor: Philips Semiconductors
                   physical id: 2
                   bus info: pci@0000:04:02.0
                   version: 01
                   width: 32 bits
                   clock: 33MHz
                   configuration: driver=pciback latency=32 maxlatency=38 mingnt=15
                   resources: irq:18 memory:ef000000-ef0001ff

I tried already to forward (00:1c.0)(03:00.0)(04:02.0) without success.

Here are the log entries from /var/log/xensource.log:

Aug 26 14:28:33 xcp-ng xenopsd-xc: [debug||40 ||xenops] QMP command for domid 14: {"execute":"device_add","id":"qmp-000127-14","arguments":{"driver":"xen-pci-passthrough","id":"pci-pt-04_02.0","hostaddr":"0000:04:02.0","permissive":false}}
Aug 26 14:28:33 xcp-ng xenopsd-xc: [debug||40 ||xenops] received QMP response qmp-000127-14 (File "xc/device_common.ml", line 502, characters 49-56)
Aug 26 14:28:33 xcp-ng xenopsd-xc: [error||40 ||backtrace] Raised Xenctrlext.Unix_error(35, "3: No such process")
Aug 26 14:28:33 xcp-ng xenopsd-xc: [error||40 ||backtrace] 1/4 xenopsd-xc Raised at file xc/device.ml, line 1591
Aug 26 14:28:33 xcp-ng xenopsd-xc: [error||40 ||backtrace] 2/4 xenopsd-xc Called from file list.ml, line 110
Aug 26 14:28:33 xcp-ng xenopsd-xc: [error||40 ||backtrace] 3/4 xenopsd-xc Called from file xc/device.ml, line 1601
Aug 26 14:28:33 xcp-ng xenopsd-xc: [error||40 ||backtrace] 4/4 xenopsd-xc Called from file lib/backtrace.ml, line 114
Aug 26 14:28:33 xcp-ng xenopsd-xc: [error||40 ||backtrace]
Aug 26 14:28:33 xcp-ng xenopsd-xc: [ info||40 ||xenops_server] Caught Cannot_add(0000:04:02.0, Xenctrlext.Unix_error(35, "3: No such process")) executing ["VM_start",["799e9021-e4e1-8824-cfaa-7c53c04b216b",false]]: triggering cleanup actions
Aug 26 14:28:33 xcp-ng xenopsd-xc: [debug||40 |Async.VM.start R:ddd6575a2d3a|xenops_server] Task 5071 reference Async.VM.start R:ddd6575a2d3a: ["VM_check_state","799e9021-e4e1-8824-cfaa-7c53c04b216b"]
Aug 26 14:28:33 xcp-ng xenopsd-xc: [debug||40 |Async.VM.start R:ddd6575a2d3a|xenops_server] VM.shutdown 799e9021-e4e1-8824-cfaa-7c53c04b216b

What can I do else to get it working?