I have just updated a beaglebone black running debian bookworm.
One of the changes was a new kernel (linux-image-5.10.168-ti-r82), but this kernel is not booted by u-boot any longer (all 4 LEDs keep glowing continuously).
Interesting enough, this kernel is just about half the size (6.6 MB) of the previous one (-r79, 11 MB).
This does not seem to be a u-boot problem, as simply reverting uEnv.txt to the previous uname_r made the beaglebone boot again.
Since this is a bbb with only 2 GB eMMC, the root file system is a compressed btrfs (which is no problem with the previous kernel, however).
Major change was…
-# CONFIG_KERNEL_XZ is not set
+CONFIG_KERNEL_XZ=y
# CONFIG_KERNEL_LZO is not set
-CONFIG_KERNEL_LZ4=y
+# CONFIG_KERNEL_LZ4 is not set
This should not have broken boot.
Debian updated lz4 tool in Trixie/sid which broke CONFIG_KERNEL_LZ4 so I had to move the whole stack back to XZ…
Can you share your serial boot log?
Regards
(I do have a 2nd Beaglebone Black, would that be of help?)
Thank you for your quick response.
I had already looked up the differences in the config-5.* (looked harmless).
Unfortunately, i don’t have any serial hardware, so until the kernel initializes the display, i am blind. But using the current kernel, the display is never initialized. 
The same happens with the 6.12.13 kernel.
Is the change of compression algorithm the reason the kernels have shrunk so much?
Xz is better at compression… But it’s much slower on a single core cortex a8 during decompression, so we used lz4 for a long time for faster boot up.
It would really help us help you tremendously if you could provide the serial output
from the 6-pin serial0 header.
Since you say you have another Beagle, you can use that to capture what’s being emitted.