Environment: SQL Server 2012 SSRS
Resulting in dialogues like this or similar message
And even though you have assigned all the right permissions on the site and additionally just to be sure on the folder but still…
Then try considering “User Account Access” issues. Try “Run as Administrator”. As it has worked for many and it might do for you too. Either on report builder or IE where you launch Report Builder from.
See this connect entry for details. Go through comments as well.
User does not have required permissions. Verify that sufficient permissions have been granted and Windows User Account Control (UAC) restrictions have been addressed