Problem is that this device wasn't decrypted after the reboot:
Aug 30 20:40:37 Server emhttpd: import 32 cache device: (sdc) SanDisk_SSD_PLUS_240GB_184302A005B3
It's strange since the device was there, but since it wasn't decrypted it can be used by btrfs, so it was like the device wasn't present and the pool balanced to single, not sure why this is happening, if you have a spare try replacing that SSD with a different one, if it still happens it's likely a bug.