Pi4b serial port

Hi, I try to connect my raspi with venus 3.52 with a Multiplus 12/500/20 via MK2.2b.
Trying to use the serial port on pins 8 and 10 of the connector prevents the raspi from booting.
Already a cable between the 2 pins causes the problem.
How can I solve it?

My serial port problem is just solved by a change in /u-boot/config.txt.

enable_uart=0
dtoverlay=disable-bt

Now, I have to check, if the Multiplus is found.

Hello. Did you make this work? I’m also trying to use a MK2.2b

Hello Tim,

till now, I did not succeed.

My next step is to make sure the cables are ok and to watch signals on RX and TX.