Same here on KDE Neon 5.12 (which is Ubuntu 16.04 + KDE stuff). Same Zoiper version.
If run with --disable-gpu flag there's a new error trying to load missing libGLESv2.so lib:
APP Started! [ PID: 27889 ]
(zoiper5:27889): Gtk-WARNING **: Unable to locate theme engine in module_path: "adwaita",
(zoiper5:27889): Gtk-WARNING **: Unable to locate theme engine in module_path: "adwaita",
APP Started! [ PID: 27924 ]
(zoiper:27924): Gtk-WARNING **: Unable to locate theme engine in module_path: "adwaita",
(zoiper:27924): Gtk-WARNING **: Unable to locate theme engine in module_path: "adwaita",
[0604/170148.089160:ERROR:gpu_process_transport_factory.cc(1029)] Lost UI shared context.
[0604/170148.331946:ERROR:service_manager_context.cc(252)] Attempting to run unsupported native service: /usr/local/applications/Zoiper5/content_renderer.service
APP Started! [ PID: 27953 ]
(exe:27953): Gtk-WARNING **: Unable to locate theme engine in module_path: "adwaita",
(exe:27953): Gtk-WARNING **: Unable to locate theme engine in module_path: "adwaita",
[0604/170148.406179:ERROR:gl_implementation.cc(292)] Failed to load /usr/local/applications/Zoiper5/swiftshader/libGLESv2.so: /usr/local/applications/Zoiper5/swiftshader/libGLESv2.so: cann
ot open shared object file: No such file or directory
[0604/170148.409569:ERROR:viz_main_impl.cc(197)] Exiting GPU process due to errors during initialization
APP Exit [ PID: 27953 ]