This case happened to me when I was deploying new environment for client, 403 is the most ugly error message you can get. There is no complete list of reasons available for this.
Error: 0
Description: The remote server returned an error: (403) Forbidden.
Severity: 5
Source: System
Debug info: N/A
Extra info: N/A
Client Source: CExProgress::ProcessPackage
Recommended Action: N/A
There is main problem for this error which is the disk space, when you export SDO file there is a temp folder in this path C:\Program Files\Microsoft Content Management Server\Server\Temp\.
While exporting there are temp files placed in this folder so you need to have space for this files or you can change this temp folder from the registry.
Adjust the following registry value and restart IIS and AESecurityService:
HKLM\SOFTWARE\NCompass\Resolution Content Server\Configuration\ResolutionTempFolder