The permissions in your screen shot look correct as long as the user account that the application pool the website is running under is part of the IIS_USRS group, if not give the user account the application pool is running under read, write, and modify permissions.
Also make sure that the application pool is using a single worker process and the rapid fail protection is disabled.
This is done by locating the application pool for the website the forum is running under in the IIS Manager, highlighting the app pool and selecting 'Advanced Settings' from the right hand menu.
Once the advanced settings are open for the app pool set the following:-
Enable 32-Bit Application = True
Maximum Worker Processes = 1
Rapid-Fail Protection -> Enabled = False