Boot Loop on my Backup-Raspi4

Hello everyone, I have a problem that I haven’t been able to solve for half a year. I use a Raspi4 4GB Model B with an original power supply, an SSD on the USB3 port.
I wanted to set up a complete back-up system (since the building technology is controlled via it, including the heating and hot water) and initially configured a second SSD. This also runs (like the first) without any problems on the (original) Raspi4.
Now I have bought a second Raspi4 (same model) and configured it to boot to SSD, then connected it to the SSD. When using this (new) Raspi4 on both! SSDs, does a boot loop occur?
I have waited for several updates, checked the Raspi power sensor (no problem), and integrations… without success.
Does anyone have any ideas before I order another Raspi4 and test it??

How to Troubleshoot Raspberry Pi Crashing.