NC 13: UI broken, recognises storage as full

My Nextcloud installation failed yesterday when for some reason, my external hard disk disconnected from the Raspberry Pi. Upon restart, it would not mount the disk anymore and forced an fsck. Now as this has been performed, the disk is mounted again, but my Nextcloud installation is completely broken.

  1. The web interface is broken, it only supplies plain text:

  2. As soon as I access any website, the RPi encounters very high CPU load.

  3. Nextcloud thinks its storage is full. It shows an according message at the top of the web interface (see picture) as well as in the logs.
    The desktop client sync is working in general, but when I try to upload new files, it fails and presents the message “Error: forbidden”.

Update: I already disabled theming as I read about a bug with that in NC 13, and then tried to clear my CSS and JS cache as well as rescanned app files as suggested by a developer. However, I performed that on the old data folder that wasn’t yet moved to an external HDD. So, I now repeated those steps on the correct folder, and now I only receive an internal server error. Unfortunately, nothing is logged though.

Can anyone help me with this problem?

Nextcloud version: 13.0.0
Operating system and version: Raspbian 8 (jessie)
Apache version: 2.4.10
PHP version: 5.6.30

Nextcloud log:

{"reqId":"QXrC1wWOu4mkpmy7xMnx","level":3,"time":"2018-02-26T19:23:27+00:00","remoteAddr":"2a02:8071:aa4:3100:7c85:d01b:e5f5:e75","user":"--","app":"PHP","method":"GET","url":"\/index.php\/apps\/theming\/styles?v=0","message":"file_put_contents(): Only 0 of 122 bytes written, possibly out of free disk space at \/var\/www\/html\/lib\/private\/Files\/Storage\/Local.php#218","userAgent":"Mozilla\/5.0 (Windows NT 10.0; Win64; x64; rv:58.0) Gecko\/20100101 Firefox\/58.0","version":"13.0.0.14"}
{“reqId”:“QXrC1wWOu4mkpmy7xMnx”,“level”:3,“time”:“2018-02-26T19:23:27+00:00”,“remoteAddr”:“2a02:8071:aa4:3100:7c85:d01b:e5f5:e75”,“user”:"–",“app”:“PHP”,“method”:“GET”,“url”:"/index.php/apps/theming/styles?v=0",“message”:“file_put_contents(): Only 0 of 342 bytes written, possibly out of free disk space at /var/www/html/lib/private/Files/Storage/Local.php#218”,“userAgent”:“Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:58.0) Gecko/20100101 Firefox/58.0”,“version”:“13.0.0.14”}
{"reqId":"QXrC1wWOu4mkpmy7xMnx","level":3,"time":"2018-02-26T19:23:27+00:00","remoteAddr":"2a02:8071:aa4:3100:7c85:d01b:e5f5:e75","user":"--","app":"PHP","method":"GET","url":"\/index.php\/apps\/theming\/styles?v=0","message":"file_put_contents(): Only 0 of 342 bytes written, possibly out of free disk space at \/var\/www\/html\/lib\/private\/Files\/Storage\/Local.php#218","userAgent":"Mozilla\/5.0 (Windows NT 10.0; Win64; x64; rv:58.0) Gecko\/20100101 Firefox\/58.0","version":"13.0.0.14"}
{"reqId":"yFJGSXltZLG4LKWcHJhV","level":3,"time":"2018-02-26T19:24:28+00:00","remoteAddr":"2a02:8071:aa4:3100:7c85:d01b:e5f5:e75","user":"Yassin","app":"PHP","method":"GET","url":"\/index.php\/apps\/files\/","message":"file_put_contents(): Only 0 of 4089 bytes written, possibly out of free disk space at \/var\/www\/html\/lib\/private\/Files\/Storage\/Local.php#218","userAgent":"Mozilla\/5.0 (Windows NT 10.0; Win64; x64; rv:58.0) Gecko\/20100101 Firefox\/58.0","version":"13.0.0.14"}
{"reqId":"yFJGSXltZLG4LKWcHJhV","level":3,"time":"2018-02-26T19:24:28+00:00","remoteAddr":"2a02:8071:aa4:3100:7c85:d01b:e5f5:e75","user":"Yassin","app":"PHP","method":"GET","url":"\/index.php\/apps\/files\/","message":"file_put_contents(): Only 0 of 121 bytes written, possibly out of free disk space at \/var\/www\/html\/lib\/private\/Files\/Storage\/Local.php#218","userAgent":"Mozilla\/5.0 (Windows NT 10.0; Win64; x64; rv:58.0) Gecko\/20100101 Firefox\/58.0","version":"13.0.0.14"}
{"reqId":"yFJGSXltZLG4LKWcHJhV","level":3,"time":"2018-02-26T19:24:28+00:00","remoteAddr":"2a02:8071:aa4:3100:7c85:d01b:e5f5:e75","user":"Yassin","app":"PHP","method":"GET","url":"\/index.php\/apps\/files\/","message":"file

Can you check your partitions:
df -h

e.g. it does not have to be your data-folder, it can be also your system disk (e.g. filled up with logfiles).