Try this for mod_security and see if it helps
http://wiki.simplemachines.org/smf/Mod_security_-_Having_problems_with_mod_security
I was quite excited for a while there...but I have tried this on both sites and it still comes up with the same error.
After reading the link you sent me I now understand exactly what the hosts have done. They have stopped linking to some file host sites. The reason I know this is because I have tested out FileFactory and Rapidshare and they are both blocked. If I put in my own web address, in place of the file hosts, then the 'Download' is completed. Whether this can be classed as legal by them as there are legitimate reasons for using file hosts i.e. for photos etc I'm not sure.
I will have to look at my options now i.e. FTP upload etc and see what to do as I'm also wondering why the uploading of large files fail in the 'Downloads' also.
I have tested out another host and the same issues are happening there too, so maybe this is going to be the norm for web hosts, only time will tell.