Possible cause is that you give the user permissions in the wrong place (under site settings) but it does not have access to browse the folders.
Solution for this is very simple and is related on how the permissions are given in reporting services, so you must check for this:
1. Access the reporting services page with an account that already has access (if the issue is with your own account, access from the machine where the report server is installed using localhost instead machine name)
2. Select the folder you want to give access (usually the Home folder) and then click on Folder Settings
3. In the security Tab, Click on New Role Assignment add the user or group and select the desired access for it.
4. Try to browse the report server with the user. You should be able to access the report server now.