This is what the plugin maker answered me:
It is a known issue that Unyson does not work with Sunshine.

OK so plugin developer agreed that it conflicts with Unyson plugin and not theme. Also it conflicts with other plugins also as described in the following topic.

https://wordpress.org/support/topic/i-cannot-open-a-galery/

https://wordpress.org/support/topic/conflict-with-gallery-plugin/

https://wordpress.org/support/topic/string-error-php/

Yes the theme functionality depends on Unyson plugin but we don’t have any control on the plugin. Also we have faced this type of issue first time that Unyson plugin conflicts with other plugins.

If you can, please also consider using any alternate plugin for Sunshine plugin.