Hum, it could be anything, really. And the only useful information you gave us is
"Writing /var/www/html/dokuwiki/conf/plugins.local.php failed"
This is quite a challenging puzzle. So i'm going to look in my
crystal ball and do some divination :
I see that this is a Debianish or maybe a redhatish system....
I see that you have shell access....
I see that you even have root shell access...
I see that you copied the files as root, not as the web user...
I also see in my cristal ball that you should run this command as root :
chown -hR www-data: /var/www/html/dokuwiki/. ||
chown -hR apache: /var/www/html/dokuwiki/.
I see in my crystal ball a big red warning :
OP is going to run the command, if it fails or if it breaks the system, it is his/her fault, not Schplurtz'.
Then I can see nothing more in my crystal ball. Sorry, the magic is gone. It's all black... and I'm tired.