I did an update to Jessie and now Owncloud is not working anymore.
When I check the error log, I see this error:
Call to undefined function json_encode

When checking if php5-json is installed, I see that, yes, it is.
When reinstalling php5-json, dpkg / apt-get tells me:
php5_invoke: Enable module json for apache2 SAPI

But I see no indication that it is.
php -i | grep json
comes back empty, not one entry mentioned in php.ini about json.

I'm very unexperienced in troubleshooting apache and php issues, so I might be missing an important detail here.
I know that Debian changed some policies concerning php json in Jessie, due to legal reasons. Can it be that Owncloud will no longer work on Jessie?

I would try to remove php5-json, restart the server then re-install it and restart the server again. Debian based distros usually add the necessary entries in the config files.

That did not work. I even purged php5-json and that did nothing, the config files are still not there. I also wiped out the whole Apache server and did a complete fresh reinstall and no dice.
Is there a way to monitor what a package installs and why it is not putting these files there?