NextGen Gallery – upload problems

I used NextGen galleries on several sites, but on one site I have a problem. I can’t upload more than one image to a gallery at a time – in fact, I can’t highlight more than one from the file location. All my sites are at the same host and they don’t have this problem.
Is this a problem of settings on my PC, or is it a problem with settings in the NextGen Gallery plug-in on my site?