Quick hack
After short investigation I've found that message is produced by action.php (dokuwiki/lib/plugins/upload)
Apparently the plugin hooks do not behave exactly as they need to in newer versions of dokuwiki.
My deep knowledge of PHP and plugins let me comment out the line 63, but no more: :-D
....
60 // check auth
61 $AUTH = auth_quickaclcheck("$NS:*");
62 if($AUTH < AUTH_UPLOAD) {
63 // msg($lang['uploadfail'], -1);
64 return;
65 }
...
The result - the message disappear, the plugin does not show up on the page.
I would guess that instead of msg() function the plugin should
a) disable Upload button and
b) output text in its place something like this:
"Your permissions do not allow to upload files."