Same issue here. I followed TEKKBEBE's guide at:
ownCloud 7 install on OMV 1 with current plugin
The issue, is the PHP session ID specified in the config.php is mangled. Take a look at:
/var/www/owncloud/config/config.php
.. the 'instanceid' line
Compare that to the config.sample.php, which has a sane session ID listed. I just replaced that long string with the one from the config.sample.php, restarted the nginx service and was good to go.
Now, i'm not sure why my logs are full of
Code
ErrorOC_Group_OpenMediaVaultFailed to get user 'joe.bishop' (code=6000, message=User 'joe.bishop' does not exist)
..but I'll track that one down and not hijack this thread