Hi, I recently ran into a problem which I'm documenting here in the hope 
it saves others some time (possibly also RC devs could be aware and 
consider logging a warning)
The symptoms are that roundcube mostly works, but hangs with a 
Saving/Sending message at the bottom of the screen if you try and 
save/send.  The browser is "frozen", in that you cannot click on other 
items on the page, and it's necessary to close the browser tab or do a 
full page reload to escape.
The resolution was to remove security headers from my default nginx config:
                 #add_header X-Frame-Options DENY;
                 #add_header X-Content-Type-Options nosniff;
I presume that the iframe option interferes with save/send process, 
hence why it breaks.  I think it's not unreasonable, however, just 
documenting the problem here since it's a common option shown as a good 
default for a tightly secured webserver setup.
Thanks to all developers for the excellent Roundcube product!
Ed W