Some more details on the ARC issue.

If I queue the message with "-oMa 25.25.25.25 -odq", the queue runner crashes as follows which seems to be the "normal" and "correct" way to crash, as would real messages coming from the Internet.

#0  0x00007f56f3ac1af9 in gnutls_x509_trust_list_deinit (list=0x55876d7fe0b0, all=1) at ../../../lib/x509/verify-high.c:213
        i = <optimized out>
        j = 0
#1  0x00007f56f3a300cb in gnutls_certificate_free_credentials (sc=0x55876d7ff2b0) at ../../lib/cert-cred.c:403
No locals.
#2  0x000055876bf9c1c7 in tls_close (ct_ctx=0x55876d5701a0, do_shutdown=do_shutdown@entry=2) at ./b-exim4-daemon-custom/build-Linux-x86_64/tls-gnu.c:3777
        state = 0x55876d5701a0
        tlsp = 0x55876c02e3e0 <tls_out>
        __FUNCTION__ = "tls_close"
#3  0x000055876bfc9c79 in smtp_deliver (addrlist=addrlist@entry=0x55876d55f988, host=host@entry=0x55876dac5838, host_af=host_af@entry=2, defport=<optimized out>, interface=<optimized out>, tblock=tblock@entry=0x55876d56f4f8, message_defer=<optimized out>, suppress_tls=<optimized out>) at ./b-exim4-daemon-custom/build-Linux-x86_64/transports/smtp.c:4850
        n = <optimized out>
        ob = <optimized out>
        yield = <optimized out>
        save_errno = 1812207714
        rc = <optimized out>
        message = 0x0
        new_message_id = "\360+sJ\376\177\000\000\000\000\000\000\000\000\000\000@"
        sx = 0x55876da3a130
        __FUNCTION__ = "smtp_deliver"
        pass_message = 0
        dane_held = <optimized out>
        tcw_done = 1
        tcw = 0
        SEND_MESSAGE = <optimized out>

The ARC crash happens because I omit -oMa 25.25.25.25 and the message is thus locally submitted. Per the configuration, DKIM signing occurs (which is not what would normally happen). The message injected (locally or with -oMa) does not have any Authentication-Results header.

2022-05-12 16:49:21 1npC09-0007sc-1D <= ***@***.com U=root P=local S=4573 id=***
2022-05-12 16:49:46 1npC09-0007sc-1D ARC: no Authentication-Results header for signing
2022-05-12 16:49:46 1npC09-0007sc-1D H=mail.gedalya.net [******]: SMTP error from remote mail server after pipelined end of data: 451 Temporary local problem - please try later
2022-05-12 16:49:46 1npC09-0007sc-1D H=mx2.gedalya.net [******] Network is unreachable
2022-05-12 16:49:46 1npC09-0007sc-1D SIGSEGV (fault address: (nil))
2022-05-12 16:49:46 1npC09-0007sc-1D SIGSEGV (null pointer indirection)
2022-05-12 16:49:46 1npC09-0007sc-1D SIGSEGV (30343 delivering 1npC09-0007sc-1D to mx2.gedalya.net [****] (<email address hidden>)
2022-05-12 16:49:46 1npC09-0007sc-1D Delivery status for <email address hidden>: got 0 of 7 bytes (pipeheader) from transport process 30343 for transport smtp
2022-05-12 16:49:46 1npC09-0007sc-1D == <email address hidden> R=dnslookup T=remote_smtp defer (-1): smtp transport process returned non-zero status 0x008b: terminated by signal 11

Running exim -q -d :

[attempt first server]
...
ARC: requesting bodyhash
DKIM: new bodyhash sha256/simple/-1
dkim signing direct-mode
...
GnuTLS<3>: ASSERT: ../../../lib/nettle/mpi.c[wrap_nettle_mpi_print]:60
DKIM [***.com] b computed: xx....xx
ARC: sign for ****.com
LOG: MAIN
  ARC: no Authentication-Results header for signing
  SMTP+> BDAT 5360 LAST

[attempt second server]
ARC: requesting bodyhash
DKIM: new bodyhash sha256/simple/-1
dkim signing direct-mode
...
GnuTLS<3>: ASSERT: ../../../lib/nettle/mpi.c[wrap_nettle_mpi_print]:60
DKIM [swdhost.com] b computed: xx....xx
ARC: sign for ****.com
LOG: MAIN PANIC
  SIGSEGV (fault address: (nil))
LOG: MAIN PANIC
  SIGSEGV (null pointer indirection)
LOG: MAIN PANIC
  SIGSEGV (30594 delivering 1npC09-0007sc-1D to mx2.gedalya.net [*****] (<email address hidden>)
)
reading pipe for subprocess 30594 (not ended yet)
expect 7 bytes (pipeheader) from tpt process 30594
LOG: MAIN PANIC
  Delivery status for <email address hidden>: got 0 of 7 bytes (pipeheader) from transport process 30594 for transport smtp

When using -oMa, "ARC: no Authentication-Results header for signing" appears for both attempts in the message log.