Steps to reproduce:

1. Enable DebugByUser by user in site.ini (e.g. settings/override/site.ini.append.php):

[DebugSettings]
DebugOutput=enabled
DebugRedirection=disabled
AlwaysLog[]
AlwaysLog[]=error
ScriptDebugOutput=enabled
DebugByUser=enabled
DebugUserIDList[]=14

2. Login as another user (e.g. editor) to the admin siteaccess - any user will do as long as it's one that has not an object ID of 14;
3. Edit for example the media folder => version 2;
4. Press preview button in the left menu - you will see the preview with debug output, which is not supposed to happen.

Note: I used an user with object ID of 14, which in my installation happens to be my main admin user. Please adapt this to your own test environment.