Unable to set up new players , please help

Bought 3 new RPi4 and tried to set them up, as usual
But the RPI gives an error about bootloader. Uploaded the bootlader, eeprom and firmware to the latest , and tried again. No luck setting up InfoBeamer.
What works - DietPI, works fully, latest Raspbian and so on.
Please help, what needs to be done, it’s urgent to set those players up!

Already responded within a few minutes to the email you wrote in parallel. Did you try the testing release?

Tried testing release, up and going.
Thanks @infobeamer-fw

testing release worked for me also, had the same challenge that the bootloader was stuck

for anyone out there, with the same problem, here is the step by step solution
download dietpi and write it on a microSD card with Raspi Imager
insert the microSd into the RPi, connect keyboard, LAN cable , HDMI cable to screen and boot it up.
after the initial and short configuration

sudo apt update
sudo apt upgrade -y

Then, write the following command

sudo rpi-eeprom-update -a

The command will show the actual bootloader and the latest available one, which we want to write on the eeprom module.
Simply issued the reboot command and after the RPI booted up, cheack again the output of the

sudo rpi-eeprom-update -a

command. The bootloader versions have to match with the latest available.
Then, and only then write the testing infobeamer .img to a microSD card and start using it.
My question is: can we later on switch to the stable channel, while we have it up and running, without any shortcoming, bootloader issues or nothing of the sort?

While that’s a way to manually upgrade the EEPROM on the Pi4, that process is not necessary once testing is finally promoted to stable (Sorry about that delay!): Both install.zip and the install.img image contain the files recovery.bin, pieeprom.upd and pieeprom.sig. On all Pi4 (not ComputeModule4) revisions this will trigger the EEPROM flashing automatically on first boot.

Once stable release 13 is finally out (any day now), you can switch back to stable in the dashboard with the only downtime being a single reboot.