I'm going off this manual which is for a BX based system. Not sure how much it applies to your system but at least its from the same company/era.
https://www.manualslib.com/manual/1427/Acer-A … 100-Series.html

Disk A error, id just temporarily set both A: and B: to none reboot and set a: back to 1.44 and see if it's happy. The fact you can boot of the drive is a good sign.

BMC and Sever Management Interface both relate to what's called Out of Band Management and allows things like Turing the machine on and off remotely.
I'd assume you can configure this in either in BIOS or a message during post along the lines of "hit F10 to configure BMC or similar"
Going off the manual above it may of been provided by the "RDM Module" which may be missing?

also pics please, love old servers 😀