Starnge serial problem with UNO

I have a problem that I can not understand, maybe you can help.

I want to read serial data from a device that is operated with an ATMEGA8.
Now I have the problem that when I try it with a UNO I don't get any data.
With a Nano it works, as well as over a USB/TTL CD2102 to a computer
The UNO I have already tested against the Nano and the CD2101 from there it receives data.
What could be the reason for this?

A wiring fault
A program fault
A faulty Arduino

Start by posting your code and a schematic

A wiring fault --> No, it's running with Nano and USB to TTL
A program fault --> No, it's running with Nano and USB to TTL
A faulty Arduino --> No, as I worte

The obvious conclusion is that nothing is wrong

but something else must be.
because still no UNO works, I tested extra another, on the TX of the ATMEGA8.
Nano and a USB to TTL work very well

there aren't any other options. That your wiring works with the nano doesn't mean anything. the connection to the nano works, that it means. As long as you don't show it to us, it can not be eliminated as a possible cause.
Without the code we can not exclude it as a possible issue.

error detected.
a 1 kilo ohm was installed behind the TX of the ATMEGA8 for protection.
Probably the RX input of the UNO is less sensitive than the Nano or the USB to TTL.