Rpi5/LE12 changes resolution up and down (HDMI)

I have a new Rpi5 and tried to install LE12

On an old TV connected via HDMI-A1, the display is not usable.

First I see the LibreElec Logo in a normal size, then after a second or so the DIsplay changes and the Logo is displayed only partially and much bigger (like in a lower resolution).

After a second it changes back to normal size and the above behaviour starts again.

The same setup on a different TV (way younger)) everything works fine.

WIthout an SD-card I can see (on the old TV) the diagnostic lines including EDID=okay

AND: sometimes the old TV shows a message 1920x1080i (but it comes from the TV)

Any solution available

seems to be better at least.

It worked with the 720 line.

Thank you