Got the bug again.  The log is silent again, after the HTTP timeout messages.  sysprof says that all the time is spent in httpGets, but I don't get meaningful stack traces (because Ubuntu builds with -fomit-frame-pointer probably).  perf top shows the same.

Attaching with gdb shows me this for the main thread:

#0  __futex_abstimed_wait_common
    (cancel=false, private=<optimized out>, abstime=0x0, clockid=0, expected=3, futex_word=0x557767016e6c <lock+12>)
    at ./nptl/futex-internal.c:103
#1  __GI___futex_abstimed_wait64
    (futex_word=futex_word@entry=0x557767016e6c <lock+12>, expected=expected@entry=3, clockid=clockid@entry=0, abstime=abstime@entry=0x0, private=<optimized out>) at ./nptl/futex-internal.c:128
#2  0x00007f1235a9566c in __pthread_rwlock_wrlock_full64 (abstime=0x0, clockid=0, rwlock=0x557767016e60 <lock>)
    at ./nptl/pthread_rwlock_common.c:730
#3  ___pthread_rwlock_wrlock (rwlock=rwlock@entry=0x557767016e60 <lock>) at ./nptl/pthread_rwlock_wrlock.c:26
#4  0x0000557766ff4a54 in get_local_printers () at daemon/cups-browsed.c:4021
#5  0x0000557766fffab5 in update_local_printers.part.0.lto_priv.0 () at daemon/cups-browsed.c:4383
#6  0x0000557766ff886d in update_local_printers () at daemon/cups-browsed.c:7103
#7  on_printer_deleted
    (object=<optimized out>, text=<optimized out>, printer_uri=<optimized out>, printer_state=<optimized out>, printer_state_reasons=<optimized out>, printer_is_accepting_jobs=<optimized out>, user_data=<optimized out>, printer=0x55776866ac00 "HP_Color_LaserJet_MFP_M477fdw_fridge") at daemon/cups-browsed.c:7111
#8  on_printer_deleted
    (object=<optimized out>, text=0x5577686e7260 "Printer \"HP_Color_LaserJet_MFP_M477fdw_fridge\" deleted by \"cups-browsed\".", printer_uri=<optimized out>, printer=0x55776866ac00 "HP_Color_LaserJet_MFP_M477fdw_fridge", printer_state=<optimized out>, printer_state_reasons=<optimized out>, printer_is_accepting_jobs=1, user_data=0x0) at daemon/cups-browsed.c:7082
#9  0x00007f1235c158b6 in ffi_call_unix64 () at ../src/x86/unix64.S:104
#10 0x00007f1235c1234d in ffi_call_int
    (cif=cif@entry=0x7fffc989bd00, fn=<optimized out>, rvalue=<optimized out>, avalue=<optimized out>, closure=closure@entry=0x0) at ../src/x86/ffi64.c:673
#11 0x00007f1235c14f33 in ffi_call
    (cif=cif@entry=0x7fffc989bd00, fn=fn@entry=0x557766ff8750 <on_printer_deleted>, rvalue=rvalue@entry=0x7fffc989bc80, avalue=avalue@entry=0x7fffc989bbe0) at ../src/x86/ffi64.c:710
#12 0x00007f1235f87163 in g_cclosure_marshal_generic
    (closure=<optimized out>, return_gvalue=<optimized out>, n_param_values=<optimized out>, param_values=<optimized out>, invocation_hint=<optimized out>, marshal_data=<optimized out>) at ../../../gobject/gclosure.c:1536
#13 0x00007f1235f81010 in g_closure_invoke
    (closure=0x5577686e76a0, return_value=0x0, n_param_values=7, param_values=0x55776870a660, invocation_hint=0x7fffc989be80)
    at ../../../gobject/gclosure.c:832
#14 0x00007f1235fae3d6 in signal_emit_unlocked_R.isra.0 (node=<optimized out>, detail=0, 
    detail@entry=3381248240, instance=0x5577686def10, emission_return=emission_return@entry=0x0, instance_and_params=0x55776870a660) at ../../../gobject/gsignal.c:3802
#15 0x00007f1235f9ba5b in g_signal_emitv
    (instance_and_params=instance_and_params@entry=0x55776870a660, signal_id=<optimized out>, detail=3381248240, 
    detail@entry=0, return_value=return_value@entry=0x0) at ../../../gobject/gsignal.c:3274
#16 0x0000557766ffeabc in cups_notifier_proxy_g_signal
    (proxy=<optimized out>, sender_name=<optimized out>, signal_name=<optimized out>, parameters=<optimized out>)
    at /usr/src/cups-browsed-2.0~rc1-0ubuntu1/cups-notifier.c:2482
#17 0x00007f1235f81010 in g_closure_invoke
    (closure=0x5577686d6b50, return_value=0x0, n_param_values=4, param_values=0x7fffc989c180, invocation_hint=0x7fffc989c100)
    at ../../../gobject/gclosure.c:832
#18 0x00007f1235fae505 in signal_emit_unlocked_R.isra.0
    (node=node@entry=0x5577686d6b80, detail=detail@entry=0, instance=instance@entry=0x5577686def10, emission_return=emission_return@entry=0x0, instance_and_params=instance_and_params@entry=0x7fffc989c180) at ../../../gobject/gsignal.c:3841
#19 0x00007f1235f9e69a in g_signal_emit_valist
    (instance=<optimized out>, signal_id=<optimized out>, detail=<optimized out>, var_args=var_args@entry=0x7fffc989c370)
    at ../../../gobject/gsignal.c:3555
#20 0x00007f1235f9e923 in g_signal_emit
    (instance=instance@entry=0x5577686def10, signal_id=<optimized out>, detail=<optimized out>)
    at ../../../gobject/gsignal.c:3612
#21 0x00007f12360e178d in on_signal_received
    (connection=<optimized out>, sender_name=0x7f121c009610 ":1.699", object_path=<optimized out>, interface_name=<optimized out>, signal_name=0x7f121c009cc0 "PrinterDeleted", parameters=0x7f121c005030, user_data=0x5577686e77b0)
    at ../../../gio/gdbusproxy.c:890
#22 0x00007f12360cd36f in emit_signal_instance_in_idle_cb (data=0x7f121c002220) at ../../../gio/gdbusconnection.c:3800
#23 0x00007f1235e8036f in g_main_dispatch (context=0x5577686aed90) at ../../../glib/gmain.c:3460
#24 g_main_context_dispatch (context=0x5577686aed90) at ../../../glib/gmain.c:4200
#25 0x00007f1235edb178 in g_main_context_iterate.constprop.0
    (context=0x5577686aed90, block=<optimized out>, dispatch=1, self=<optimized out>) at ../../../glib/gmain.c:4276
#26 0x00007f1235e7fbdf in g_main_loop_run (loop=0x5577686e3ef0) at ../../../glib/gmain.c:4479
#27 0x0000557766feb3ff in main (argc=<optimized out>, argv=<optimized out>) at daemon/cups-browsed.c:14077


Some background details: both times this happened it happened when I resumed my laptop after coming to work.  There's one network printer in the local LAN, and also a Samba server that exports a print queue (for the same printer).  I've never touched IPv6 and I doubt it works, outside of whatever local autoconfiguration happens with stock Ubuntu installs.