Versions of relevant programs:
- MacOS 10.15.7 "Catalina" (I know, I know...)
- AFNI 2021-03-24
- Xquartz 2.8.0

In one terminal, with the following environment variable modifications:
export AFNI_REALTIME_Registration=3D:_realtime
export AFNI_REALTIME_Base_Image=2
export AFNI_REALTIME_Graph=Realtime
export AFNI_REALTIME_MP_HOST_PORT=localhost:53214
export AFNI_REALTIME_SEND_VER=YES
export AFNI_REALTIME_SHOW_TIMES=YES
export AFNI_REALTIME_Mask_Vals=ROI_means
export AFNI_REALTIME_Function=FIM
I launch `afni -rt`, which successfully loads the standard AFNI viewer.
In a separate terminal, I run
`rtfeedme FILE`
and no matter what I file I send, I receive the following error message:

++++++++ IMAGE SAVE SETUP WARNINGS ++++++++
++ Can't find program ffmpeg for Save to MPEG-1
++ To disable these warnings, set environment
++ variable AFNI_IMSAVE_WARNINGS to 'NO'.
+++++++++++++++++++++++++++++++++++++++++++++
. Widgets...... Input files:+/Users/tevesjb/AFNI
session # 1 = tevesjb/AFNI/ ==> 54 datasets
dataset count = 54
Time series = 0 files read
.[tc]sv data = 4 files read
Path(s) to be searched for plugins:
/Users/tevesjb/AFNI /Users/tevesjb/AFNI/../lib
++ AFNI is detached from terminal.
(base) tevesjb@MH02217074MACLT AFNI % -- no T2star_ref from env

Plugins = 49 libraries read
RT: AFNI realtime plugin is active; cf. README.realtime document
RT: starting to listen for control stream.

++ NOTE: This version of AFNI was built Mar 24 2021 ++
++ NOTE: 'Define Markers' is hidden: right-click 'DataDir' to see it
++ NOTE: Use '-seehidden' option to see which plugins are hidden
RT:---------------------------------------
RT: connected to control stream 127.0.0.1
RT: version 23 Jan 2020: ROIs_and_data
RT: init RT input via control channel
RT: waiting for data stream to become good.
RT: receiving image metadata=164 bytes
RT TIME (starting to receive realtime data): 2261 sec, 175 ms (0 ms passed)
## PARSE RT Detrend: RT Options mode 0x0 polort -1 fwhm 0
Has shared memory buffer gone bad? shm_nattach[shmctl]: Invalid argument
RT: finish channel 01 with 1 bricks completed.
RT: cpu time = 0.02 elapsed time = 0.29
RT: sending dataset rt.__001+orig with 1 bricks
to AFNI[A], session /Users/tevesjb/AFNI/

Fatal Signal 11 (SIGSEGV) received
AFNI_autorange_label
AFNI_reset_func_range
AFNI_setup_viewing
AFNI_initialize_view
AFNI:main
Bottom of Debug Stack
** AFNI version = AFNI_21.0.21 Compile date = Mar 24 2021
** [[Precompiled binary macos_10.12_local: Mar 24 2021]]

Additional notes:
I checked previous notes, and saw that DYLD_LIBRARY_PATH can be an issue, so I double-checked that environment variable:
:/opt/X11/lib/flat_namespace

Any ideas what might be going on?