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?
Thanks for reaching out to Adobe Community.
The mboxDisable temporarily disables the target tests on your site to see how the default site looks. Do these pages need authentication or some sort of information to be filled? or are they on https
Remove these parameters and then create the activity.
Hope that helps!
Turns out the answer was to check the box in settings to "Enable Enhanced Experience Composer". I think we are OK now.