Notifications

The error looks like something coming from clicking the Follow button, so not sure how that is related to the notification field.

Yes, the UI for notifications changed to merge the 3 old fields into one, but you should still be able to select any Group or Team that has access to the content. But you can only do this if you have Edit access to the node. So you'll need to give a more step-by-step procedure for reproducing any problem on a clean Atrium site as the notifications for groups and teams seems to work fine on our test sites here.

Even as the administrator user, be sure you are added somehow as the member of the space and are the Admin of the space (either directly or via a parent space for inheritance). The Organic Groups module restricts even what the Drupal Admin user can do unless you are added to the space membership. If you created the space as the Drupal Admin then you should already have ownership and edit access.

The error message you displayed also seems to mention "fullcalendar_create", which sounds like you have clicked on the calendar and are trying to add a new event from the popup dialog. Maybe there is a problem with this popup. Try using the normal Create button in the lower-right corner of the screen to create new content.

It would be helpful if you can actually post the exact step-by-step procedure for getting the error on a fresh site (starting from how you created Groups, spaces, etc)

Thanks for the steps. So on step #2, instead of double-clicking the calendar to create a new event, can you select "Create Event" from the + button in the toolbar (bottom-right editor bar if using the new theme) and then see if you can enter the "group" there?

Not sure what else to suggest then. I followed your steps on a fresh Atrium site on Pantheon and did not have any errors or problems, so this is somehow specific to your site.