Appointments+ causes random internal server errors, can't access admin area

Since the site is inaccessible (internal server error), I can't select the plugin under the site name because the list isn't loading. The site is

I had initially activated Appointments+ and it had a conflict with W3 Total Cache, causing an Internal Server Error when editing/adding posts/pages only. I disabled Total Cache. I enabled some 'add-on's in the Appointments+. These add-ons didn't work properly and some didn't even appear in the settings. Also the General Settings tab in Appointments+ got corrupted or something, because the fields got cut off right after the section about generating the appointment page... by cut off i mean that nothing below that appeared, including the save changes button. I tried to enable the google map locations add-on in appointments+ and the entire site crashed now. I am aware that this site is installed on top of another WP installation, and that could be causing the issue, but it is only temporary for development. Could this be a memory limit issue?