I just tested it on Windows with a freshly built Geany from master and it works as expected. Then I used the last nightly build and it failed as described.
So, it is indeed an issue with the nightly builds. For whatever reasons.
@Heihon if you want to test and confirm my assumption, I created test installers from current GIT master: http://download.geany.org/snapshots/geany-1.29nightly20160918_setup.exe http://download.geany.org/snapshots/geany-plugins-1.29nightly20160918_setup.... These include also updated translation files.
Btw, with the cross-compiled nightly builds I get also warnings like ` Error loading theme icon 'geany-build' for stock: Unrecognized image file format`. So I assume this is really some sort of binary incompability, probably between the Geany binaries and GTK/Glib.
As said, at some point I'll change the way the nightly builds will be created and then they should get more usable again.