Passat 3BG: No Access to Control Units

Hello,
I'm having the following problem:

"I'm unable to perform a diagnostic check because the diagnostic device isn't finding any control units. The error code 1551 indicates something about the K-line, possibly related to the positive power line. I've already measured the cables at the diagnostic connector, both the black and red ones, and both are receiving power when the ignition is on."
Regarding the radio, nothing has changed; it still has the original MFD radio.

What could it be! Does anyone have a clue? I'm at my wit's end.

Okay, one more thing: I already went to a VW dealership, but they couldn't read the data using a Gutmann tester. Then I went to an independent workshop, and they were somehow able to access the data using what I think was a Siemens test device.

Or could it be due to the mass electrodes!

Fuse okay? Disconnecting the battery for a short time might also help.

With VCDS, you may be able to see existing shortcuts.

"The fuses are all fine! I'll disconnect the battery again and try again on Thursday. Any further tips would be helpful!"