It is 11 years old asus p8z77-v
For a long time I have two SSD-s connected to it. Both are bootable, have GPT partition table, and EFI partitions with boot and esp flags. There is Debian installed on both. Bullseye on the first one and Buster or maybe even older on the other. No issues until today.
What I did today: I removed everything from the case (in the process of building new machine), then assembled everything on bench for temporary usage. The only change is different choice and order of sata cables plugged into mb and CDROM not connected at all. There is also 3rd HDD - not bootable (I had 4 sata devices connected and now have 3 of them)
And now my motherboard refuses to boot from any of these two SSD-s. Moreover it doesn't show them for boot selection when "EFI only" is selected from compatibility-related menu in bios. Seems that it thinks they are both MBR ones and boot attempts result in "reboot and select proper boot device" message. I believe it is very unlikely that two separate drives got corrupted the same way. On the other hand I'm able to boot from usb stick, look at them in gparted, mount partitions - everything looks good (I see expected files on EFI partitions in particular). It seems that my motherboard just went mad, maybe for some security-related reasons. I'm thinking about reinstalling grub on these drives, but I'm afraid of things getting even worse.
So... the general question i why and what else could I do?

ardabro,

Try connecting only one SATA cable from the SATA0 header on the motherboard to one of the SSDs.

Are all other connections correct?

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UEFI stores data in NVRAM on the motherboard - including device enumeration order. Move stuff around, this is gunna happen. Have a look at the output from "efibootmgr -v" run from the liveUSB.

I'd just chroot into each system and re-install grub to the disk. Then you can use efibootmgr to delete the old entries.

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The Asus P8Z77-V provides two SATA controllers, a six port controller integrated in the Z77 Express chipset and a separate ASMedia two port controller.
In some cases, Linux kernel doesn't provide same disk order as printed on mainboard and in manual. Connection of the third disk drive may derange disk order and causes boot fail.
Start with one SSD at port SATA6G_1 and check if system boots.
Connect second SSD to port SATA6G_2 and check if system boots.
Connect HDD to port SATA3G_3 and check if system boots.
If last check fails or HDD is a SATA 6.0 GB/s device connect HDD to port SATA6G_E1 and check if system boots.
Note: If your HDD is a SATA 6.0 GB/s device and it must run at a SATA 3.0 GB/s port in your setup its performance loss is nearly negligible in most cases.
If HDD uses MBR and CSM cannot be disabled try it with GPT on HDD.

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Thank You guys!
Indeed: my motherboard lost all nvram boot entries. I managed to restore them with efibootmgr -c and didn't even need to reinstall grub.