Several methods no longer exist. Patching just the pkg-config file doesn't suffice this time. Implicit declarations lead to the plugin failing to load.
From 0.10 on, libchamplain needs GTK+ 3. As GTK+ 2 and 3 cannot be mixed in the same process, support for libchamplain >= 0.10 is only possible once Claws Mail works with GTK+ 3.
Confirmed. I've made the same discovery meanwhile.
Thanks for the quick reply, Holger!