@JorgeB ok the exact same thing happend today as well, wenn I tried to connect the SSDs to a tray and pushed them into the hot swap bays (PC was off) and I used other bays as the last time. The main cache SSD wasn't identified, which I only realized once unRAID was booted, but the array was not started.
I did a shutdown and tested the trays by going directly into the BIOS and not booting into unRAID.
In the end I connected the difficult SSD directly to a SATA port and didn't use the backplane of the front hot swap trays.
Now the main SSD was not listed as a cache, it was an unassigned devices and only the 2nd SSD was still listed as the 2nd SSD. After adding the unassigned Device SSD as the 1st cache disk, it got the "blue" status with the remark "All existing data on this device will be OVERWRITTEN when array is Started".
Is the cache/BTRFS unhappy about a missing drive and refuses it after adding it again? I am a bit lost about the behaviour of unRAID... I will be more patient and hope for a reply
unraid-diagnostics-20210126-2125.zip