Two main things I would look at:
However, the GTK3 package change doesn’t look obviously “strange”, it just enables debug flags:
Unless that’s providing more output and something is interpreting that extra output as an error, or that error is actually unrelated or a symptom.
This bug report claims it’s GTK3:
So, first I’d try rolling back GTK3 to the previous 1:3.24.28-2 version.
Do any other applications crash, or is it just Thunar?