I think this may be related to some appended URL parameters. With some
URLs I am seeing a parameter like ?_AT_Admin=1&mboxDisable=1 being
appended to the URL and our setup redirects all mixedcase parameters to
lowercase, which I think is sending the preview pane into a never ending
loop. Any option to change that in Target settings anywhere?
When selecting the URL for an Activity, certain chosen URLs will cause
the preview pane to refresh constantly. Other URLs on the same domain
a known bug?