clicking Junk button causes RoundCube to stop functioning

[emailAdm1n]

While in Inbox , preview pane open, click the Junk icon . Message gets 'junked' but error shows up in preview pane:
Forbidden

You don't have permission to access /roundcube/ on this server.


browser popup message of "Loading ...." keeps flashing but nothing can stop the process except for exiting the browser.

(doing Junk via popmenu does not cause problem)