Server side error. What emulation mode are you running on your BB browser?
Open browser > Options > Browser Config
If it's not set to Blackberry emulation, try that. If it is already, you may benefit from another emulation mode IF you're only talking about one particular site you've having problems with or perhaps a 3rd party browser in those instances so you don't have to switch modes.
Another thing to try is clearing your cache. That's been reported to work with some 500 errors.