Ran the debug session as requested. Some comments:

At 00:21:29 I executed "syncevolution --quiet --print-config memotoo" in
another window
At 00:22:57 I executed "sync-ui" in another terminal, and exited the GUI a
couple of seconds later.

After control was initially returned to gdb, the process was still not dead, as
evidenced by "ps" in another window. I issued "contiune" to gdb, after which it
eventually died.

I notice that the daemon complains about the source "autosyncinterval=10m".
This source appeared in the config file as a section [autosyncinterval=10m]
after earlier today I had executed both:
$ syncevolution --configure memotoo autoSyncInterval=10M
$ syncevolution --configure autoSyncInterval=10M memotoo
in attempts to learn to set that interval.

Could that be relevant? How can I safely get rid of it (manually editing files
under .config/syncevolution seems unsafe to me)?