I also had a at+qmbncfg=“list” FAIL error updating the BG96 firmware. Even with Version 6.0 of QFlash.
After renaming the folder name in the FW update package from “update” to “mbn”, QFlash starts updating the modem but has error at+qmbncfg=“add”,“acdb.mbn” FAIL after reaching 100%.
See also my post here.