Installed backup app and now "Internal Server Error"

Typically I bug my system admin but I think it’s a good idea to learn and know this stuff. I had almost-the-latest version installed and running (I don’t know the version), but I installed a backup app (I think this one: Backup - Apps - App Store - Nextcloud), and now it’s inaccessible. I get this message when entering the URL:

Internal Server Error

The server encountered an internal error and was unable to complete your request.
Please contact the server administrator if this error reappears multiple times, please include the technical details below in your report.
More details can be found in the server log.

I think the drive is full because this sort of thing has happened in the past. So now I need to SSH into the machine and check if the drive is indeed full. Is that correct?

Loading the URL now brings up this:


It looks like you are trying to reinstall your Nextcloud. However the file CAN_INSTALL is missing from your config directory. Please create the file CAN_INSTALL in your config folder to continue.