I found the reason now. I went to capture some images inside the house using flash, and came back to try the remote shooting once more. To my surprise, the button was now active. The reason for this is not that I used the flash, though: for most of my images, I enable mirror lockup and use a remote control (a tripod as well) - but when using a flash, I don't use neither the MLU nor remote mode. And apparently both of these prevent the use of the capture button on the laptop screen... even though some of us might think the remote mode should be selected when using the laptop, and that locking up the mirror shouldn't be much of a problem either. A third setting preventing the PC shooting is the Bulb-mode. The laptop screen won't even let you select it, but you can still do it by turning the camera knob, in which case the button turns gray again. I would have thought that using the PC to control the shooting would at least let you have a custom exposure time, even if manually controlled, but looks like not: you are still limited to 30 seconds max. Why not an option for a custom exposure duration? So as far as I know, the only option is still a purchased remote control, or keeping the camera button manually depressed. (unless there's another version of the EOS utility software with added functions I haven't heard of) Maybe this will help someone. At least I thought I had tried it all, but now it was a combination of two settings.