Downloads halting (possibly phpBB specific)

I’ve had a number of my forum users complaining that downloads both from my site, and via the phpBB forums, have a tendency to get interrupted after a while.

As with all network related issues it only affects some users, and doesn’t affect me at all (though I’ve encountered the same thing plenty of times on other sites).

Many of the affected users are reporting errors such as “the server does not support resuming downloads”, so I expect the problem may be that any number of internet related gremlins has interrupted the download, but when their browser tries to resume it isn’t able to.

I don’t suppose anyone can help me diagnose the problem or figure out what’s wrong? I’m particularly curious to know if anyone else with phpBB forums that have many attachments experiences similar complaints?