Most likely this is not a BeagleBone specific thing, just happened to me, and I'm really confused, probably missing something obvious.
There are no problems with 4.14.51 kernel booted without any initramfs (initrd.img).
However, in certain setups I have to boot in rootfs in a raw image on a FAT partition, thus I prepared an initrd.img using the standard Debian tools.
The overall boot process is fine, everything works except loading modules. I get:
"modprobe: ERROR: could not insert <module>: Exec format error"
I tried including modules in the initrd.img, but they are not loaded either.
Thanks for advises and directions!
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Sergey