Upgraded to 8.1 today and the issue remains.
So I set up a Debian Wheezy VM just to test this, and was able to reproduce it straight away: did a clean install of 8.1 in /var/www/owncloud/ and set the data directory to /var/www/oc (whose owner I set to www-data:www-data). After installer was finished, selecting Admin from the Admin menu immediately causes ”File does not exist: /var/www/owncloud/data” to show up in error.log.
There was also this other error associated with it (copying here by hand instead because haven’t set up copy & paste in VM yet):
touch(): Unable to create file \\/var\\/www\\/owncloud\\/data\\/owncloud.log because No such file or directory at \\/var\\/www\\/owncloud\\/lib\\/private\\/log\\/owncloud.php#53
It would seem to hint the source of this being OC_Log_Owncloud::init (which is where the line it refers to belongs to AFAICT). For some reason this message about touch() failing does not show up on my actual installation’s logs (the one I pasted the logs from in the initial report above). It no longer seems to show up in the VM either after I raised logging level up to ”Errors and fatal issues”; not even after I lower it back down to ”Info, warnings, errors and fatal issues”; only the ”File does not exist: /var/www/owncloud/data” still keeps appearing. Perhaps the vanishing of that other message is also caused by the same problem with OC_Log_Owncloud::init.