[EDIT: solution to "RFM stuck in a reboot loop after updating to mypinballs game code v2.24/v2.50 found]
I've been trying to get RFM/nucore to use the mypinballs 2.50 updated code. I followed the instructions from the 'update_help' page on the mypinballs website to place the update rom files in the appropriate folder, and delete the nvram files. So far so good. However, when I reboot the machine, it gets stuck in a reboot loop. The game gets to the white screen with a few lines of blue text about checksums (last line is something like 'running update' - cant really read it because it flashes by so quick). And then... nucore just reboots. I tried v2.24 - same results.
I have done some digging around, and eventually got the below info from the nucore log file. Note that game code v2.50 seems to be recognized, but during startup there seem to be some non-fatal errors, and a fatal one "system reported reboot" - and then the process starts all over again.
After copying the nucore folder from my RFM machine into a virtualbox VM I was able to replicate the behavior in Virtualbox.
The game works fine with the v1.8 update roms that came with pinbox. Does anyone have any insights on what may be the issue here?
The solution: update the game with v1.8 roms. Start the game. This will recreate the nvram files. Then proceed with updating to mypinballs v2.50, but counter to the instructions, do not remove the nvram files. For me this resulted in the update working as expected (both in VirtualBox and on the machine itself). Yay! Next step: install shaker motor 
Log file snippet
------------------------
Nucore Launcher V1.0
**************************************************************************
WATCHDOG INIT
Nucore USB update 2.0 running
-----------------------------------------
Checking for file updates
no update to n_update
no files to delete
-----------------------------------------
Done
<SKIP>
**************************************************************************
Nucore - Nucore version:2.25.3aR Copyright Big Guys Pinball LLC 2010, 2015, 2016
EMU Version: 2.25.3aR
Build Type : RELEASE
Build Date : Mar 26 2018 19:04:42
<SKIP>
Auto detecting playfield.
AUTO DETECT PLAYFIELD
Playfield:rfm_15
<SKIP>
Nucore IPL
**************************************************************************
Loading all ROM data...
nvram1 ok
Using Real Parallel Port
Initialization complete!
Starting Emulation....
***RESET***
[50070] Revenge From Mars - Version 2.50 -- ( Flash V2.50, Rom V0.80 )
**************************************************************************
getting real addresses
connecting to server 255.255.255.255, 2069, 2070
unable to send test message to server
Pin2000 Option ROM bootstrap loader - V3.4 - Thu Apr 01 09:30:02 1999
Probing the PCI bus...
Cyrix MediaGX PCI device # 0:
VID 0x1078 DID 0x1 (status 0x280 class code 0x60000 rev 0x0)
WMS PRISM PCI device # 8:
VID 0x146e DID 0x1 (status 0x280 class code 0x30000 rev 0x2)
Cyrix Cx5520 PCI device # 18:
VID 0x1078 DID 0x2 (status 0x280 class code 0x60100 rev 0x0)
WMS PRISM serial number: -1
Watchdog running
[ VALIDATING UPDATE BOOT DATA ]
Boot data structure:
boot_data_timestamp: "Fri Dec 16 14:49:46 2022"
boot_data_checksum: 0x3d5e6c9c
boot_data_version: 1.1
boot_data_size: 8192
im_sys0_size: 112651
im_sys0_checksum: 0x8558487f
system_version: 1.36
game_id: 50070
game_version: 2.50
game_reloc: 0x100000
game_code_size: 655232
game_code_checksum: 0xe260f95f
symbols_size: 196992
symbols_checksum: 0xd8bb5f6
im_mask0_size: 3045554
im_mask0_checksum: 0x9f1410bd
im_mask1_size: 4194304
im_mask1_checksum: 0x40f764fb
im_mask2_size: 4194304
im_mask2_checksum: 0xb1f84431
im_mask3_size: 4184362
im_mask3_checksum: 0x5745869f
magic_cookie: "13Dec96"
boot_state: 1
[ VALIDATING UPDATE SYS IMAGE ]
[ VALIDATING UPDATE GAME CODE ]
[ VALIDATING UPDATE SYMBOLS ]
[ STARTING UPDATE GAME CODE ]
XINA: V1.36 - Sun Oct 30 17:16:00 2021
(C) Copyright 1997-1999 Williams Electronics Games, Inc.
XINU: V7
Symbol Table Information: update 19038 symbols @ 0x122f5e2c
*** NonFatal: 1 Jan 1999 12:00:00
*** NonFatal: Last[XPid -1 APid -1 (none)] Current[XPid 0 APid -1 (prnull)]
*** NonFatal: Retrieve Resource (get &
Failed, ID=
*** NonFatal: 1 Jan 1999 12:00:00
*** NonFatal: Last[XPid -1 APid -1 (none)] Current[XPid 0 APid -1 (prnull)]
*** NonFatal: Retrieve Resource (get &
Failed, ID=
Allegro Graphics Library: V3.0 1997
ez0: port 0x300 irq 7 mac 00:00:c0:01:02:03 type SMC8216T (8 bit)
*** Fatal: 11 Jun 1999 13:08:31SYSTEM REPORTED REBOOT
*** Fatal: 11 Jun 1999 13:08:3
MAIN SHUTDOWN
NUCORE SHUTDOWN
0 frames in 0.00 seconds = -nan FPS
AUDIO SHUTDOWN
WATCHDOG SLEEPING DURING SHUTDOWN
LAUNCHER[5] Restarting Nucore.
Update_contents_just250 (resized).jpg