RN104 failure - reboot/power cycle when any disk is in any bay

Last night Plex stopped picking up media stored on my RN104, and when I checked it this morning I noticed that the display on the front of the NAS was flickering on and off, briefly showing some sort of boot message in the milliseconds that it was on.

Obviously in that state I couldn't do any of the normal things I'd do to check on it (log into the web admin panel or NAS Utils on my phone).

I tried flexing the power adapter cable joints, since the display was flickering as if it was a bad connection, but that doesn't help (nor does switching power socket).

I've tried a long power off then back on: no joy.

I've tried removing individual disks: no joy. Any existing disk left in cause the same behaviour.

I've tried removing all disks and adding a brand new disk. Same behaviour.

I've removed all disks and allowed it to boot into safe mode. This works fine, allowing me to log into the web admin panel (at least to the Setup Wizard) using the default admin user/password, which I assume is the expected behaviour. The NAS also then shows up in RAIDar as healthy.

Any ideas what to try next? I've ordered a replacement power adapter, in case any draw on the power from disks is causing an issue, but that seems like a long shot. Replacing the NAS chassis would be a financial pain, but if its necessary then not a problem - this one has lasted for a long time. I've got cloud backups of anything particularly valuable, but ideally I'd like to recover the data. The disks were in XRAID; NAS is running 6.10.5.