The files in /ws-export/var/www-data are not being deleted after being sent to the client, and are filling the disk space unnecessarily.

find /ws-export/var/www-data -type f | egrep -o "\.[a-zA-Z0-9]+$" | sort -u | xargs -I '%' find . -type f -name "*%" -exec du -ch {} + -exec echo % \; | egrep "^\.[a-zA-Z0-9]+$|total$" | uniq | paste - - gives:

.css    300K    total
.epub   195M    total
.lock   0       total
.log    50M     total
.map    4.0K    total
.meta   32K     total
.mobi   162M    total
.pdf    2.7G    total
.php    796K    total
.rtf    1.1M    total
.sphp   268K    total
.txt    3.5M    total

They've been deleted manually for now, to get things running again.