This will either be a CDO.dll issue or permissions. Seeing as your BES is 4.0 SP3 and your Exchange is 2003 possibly with SP2.
If your Exchange is 2003 SP2 then your BES needs to be at least SP4 to support. There are issues with the Calendar if your BES is not at SP4 or higher when in an Exchange 2003 SP2 environment.
If you check your cdo.dll on both the BES an Exchange and they match then this is probably your issue.
Regardless your BES should be at SP4 if you are running Exchange 2003 SP2. In the instance that your Exchange is 2003 SP1 or lower then check the cdo.dll versions and permissions for the BESAdmin account. It's one or the other.
You could always try adding a new appointment and then check the MAGT logs, search from the bottom up for CDO and you will see events for that user. They will let us know what the issue is.
Just search the BlackBerry Technical Solution Center
for "one way calendar".