Issue solved! This is not a request for support, only intended to help someone who is perhaps facing the same issue I was.
My server has an internal slot for an M.2 PCIe nvme SSD that I use as a cache for the appdata, domains and system shares. The server is on standby during nights using the Sleep plugin. After a wake-on-LAN ping from my iPad in the morning, the cache drive failed to mount. I saw log entries saying I/O error on dev/loop2, and mc displayed the device name in red with a question mark under /mnt. A reboot always solved it, until the next WOL. Other posts here made me double-check that my docker apps were correctly configured with all folders in place, then I suspected a hardware issue. My SSD is less than a year old, but it appears that some controller chips don't support sleep/wake modes. I replaced the SSD with a different make & model, and now the issue is fixed!
The SSD that didn't work is a GAMMIX S50 Lite 512G-CS. The replacement is an Innovation IT SSD M.2 256GB.
Cheers,
T