All my users receive the above when trying to download anything over 100k or so via their BlackBerry browser. It appears there used to be a fix for this:
Browser Error - PinStack.com - Blackberry Forums

Unfortunately that seems to apply to an earlier version of BES as I don't have the option described there. I am using BES 4.1.3.18, and if anyone can help me with this I will be extremely grateful.

The option is still there; they've just moved it along with the other MDS options to the green "MDS" server instead of the blue "BES".

You'll have a green server on the left of the Blackberry Manager screen called "<something>_MDS-CS_<number>". Click that, edit properties, General, and you'll see "Maximum KB/Connection".

There are also maximums for individual file types too.