You can just do a generic journalctl > log.txt to catch everything and grep through it looking for. Sometimes if I know I can trigger an error I use -r which shows newest entries first. Then I can quickly hit CTRL-C if something shows up since you see it a bit longer. One can also use specifiers to limit to systemd units (-u) or SYSLOG_IDENTIFIERS (-t). and of course, use -g to sepecify a grep pattern (perl style).
EDIT: -g is not supported on device.
If desperate strace 
I have to admit I’m not the best at this …
It looks like he’s creating exra logs. But for the sake of simplicity, if the bug is also showing up for the Jolla Gallery, it makes sense to focus on that since it will be familiar to @mal & co. @mal, you any advice for capturing logs for debugging this?