I ran into this problem multiple times now. Unfortunately, it occured with a considerable delay, so I had forgotten about the solution the second and third time around.
The error is, that you click on a PDF-file in your owncloud web interface file listing and want to view it. But no luck – the viewer only gives you an error message.
Like, the error is caused by an Apache module called mod_xsendfile.
On my synlogy NAS it has a config file located at: /etc/httpd/conf/extra/mod_xsendfile.conf-user
This you need to edit to contain the following:
LoadModule xsendfile_module modules/mod_xsendfile.so
XSendFilePath /var/services/web /var/services/homes
SetEnv MOD_X_SENDFILE_ENABLED yes
Especially the last directive is key to making owncloud work again.
Now restart the webserver with:
httpd -k restart
Hope this saved you some time. It will for me the next time I encounter this error. 😉
PS: found this fix at: https://forum.owncloud.org/viewtopic.php?f=26&t=21036