Hi,
There was an issue in previous versions of mojoPortal where certain types of errors could trigger the ip address to be banned, specifically path too long exceptions.
Once the ip address is banned it will result in the 403 forbidden response. In the latest version of mojoPortal, we no longer ban ip addresses for this type of error because it can happen under normal circumstances.
You can disable the BannedIPBlockingHttpModule by commenting it out from the <httpModules section and the <modules section in Web.config
Then you can go to Administration > Advanced Tools > Banned IP Addresses and find your address and delete it from the list of banned ip addresses.
The error in your log sounds like the file /Default.aspx does not exist on disk, it should exist. I would check and make sure it has not been removed or renamed.
Hope it helps,
Joe