By some miracle I built a legacy kernel 5.10.160 with RBD support. I can't say exactly what exactly helped, but I did the following:
1. I literally added output/config/linux-* and config/kernel/linux-* to every file:
CONFIG_BLK_DEV_RBD=m
2. Added to ./config/boards/orangepi5-plus.conf
MODULES="rbd"
3. Repeated previous build instructions.
And on this kernel the OPie5+ boots without problems every time.
@LoneExile Thanks for the answer! Unfortunately, I need the Debian version of the distribution because I'm installing Proxmox. I discovered that the default build bookworm_edge_6.6.0-rc1 contains the RBD module. But here I encountered the surprising behavior of the Orange Pi 5 Plus board: now it boots not every time. I literally have to switch the power supply several times until the OS boots. This does not happen on the bookworm_legacy_5.10.160 kernel version. I have three identical Oragne Pis with different power supplies and this problem is reproduced on the bookworm_edge_6.6.0-rc1 kernel. But this is a question for a another thread. Nevertheless thank you!
Dexogen got a reaction from rinseaid in Trouble Building and Loading the `rbd` Module with Armbian Kernel
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