I have same problem with 600SEL. Both EGRs and pipes were cleaned, but the fault comes back. I don't think the pressure drop at idle is relevant. We need to check it during the test when all conditions met.

Here is fault P0 400 description copied from the MB OBD2 technical training notes:

Activation requirement Check Engine Light: After one "performed" test with failure, at start of next trip.

Test to detect fault:
The EGR logic chain is checked if the following conditions are simultaneously satisfied:
- No failure in the electrical activation of the EGR switchover valve Y27;
- Deceleration fuel shut off operating;
- Close throttle position recognized;
- Engine speed approx. 1900 - 2600 rpm and constant;
- Intake manifold pressure less than approx. 325 mbar;
- Momentary intake air temperature >10C

After approx. 1 second, the diagnostic module determines the intake manifold pressure. Initiated by the diagnostic module (via the CAN data bus), the engine control module then briefly activates the EGR switchover valve. The intake manifold pressure is evaluated for approx. 3 seconds.
The pressure in the intake manifold must drop by at least approx. 15 mbar while the EGR switchover valve is activated.

The EGR valve is opened with vacuum and closed by internal spring pressure. This code can not only be set by the failure of vacuum to reach the EGR valve but by a mechanical failure of the EGR valve itself. The valve must fully open and close with an effective seal.
Check vacuum line integrity, switchover valve mechanical, EGR valve vacuum and mechanical, EGR passage ways for carbon blockage.