TTL-input doesn't show anything on screen with my Z80-retrocomputer

[Perdita-Olio]

Like I wrote in the title, nothing shows on screen when I connect my Z80-retrocomputer to the Versaterm.
Tried different settings, nothing helps.
The Z80-retro-PC works well on the ASCII VT100 video terminal Ver. 1.3 of Geoff Graham.
When I connect the RX and TX of the Versaterm the screen shows what I type in, so far so good.

I made an inverter between the TX from the Z80 and the RX of the Versaterm.
At that moment I get some rubbish on the screen, every time when I press the reset-button of the Z80-board the screen shows almost every time the same string of different figures.
Changing the settings will not help to make it readable. I know the baud-rate, the number of bits etc, the same settings as in the ASCII video terminal, but still no readable text.

I tried the latest firmware, I even took another RPico, looked for short-circuits, all the components on the right position and in the right way, especially the semi-conductors like the diodes.

I'm out of ideas. Can somebody give me a suggestion on what to look at?