Since I haven't received any replies, I am adding some extra information, hoping that someone comes to my rescue.
CHIPSET : cpu 68020, turbo none, video pal, chipset AGA, cd32 off
MEMORY : CHIP 2.0, slow NONE, FAST 24 MB (try also with 4 MB), ROM
kick3.1, HRTMON disabled
VIDEO : lores / hires none, scanlines dim, dither SPT
Hardfile Workbench 3.9
And yes, firmware says ATH160221. I am using the file firmaware.bin and firmware.upg posted on February 21 in the Atari Forum. I have copied those two files, plus the new core.rbf to the root of my SD card. Then from the OSD menu I navigated to the Firmware & Core option and Update.
After booting it shows that I'm running v1.2.
Honestly, I am not so sure that I am doing the upgrade process correctly. This is the first time since I bought the MiST that I am doing an update.