The exact error message does vary a bit due to the fact that the actual name of the torrent is included in the error message.
Regretfully I don't recall the message well enough for me to post it at this time. If (when) I see it again I'll make a note of the exact wording and reply with it at that time.
As to what happens, I'll click the OK button (only option), then the error message closes.
Next another message stating that I don't have the rights to write to the temporary directory where the .torrent file is placed. Again, my only option is to click the OK button, which, again, makes the message close.
At this point, I'm back at the page where I tried to start the torrent download and can try again.
Usually I have to go through this process two or three times before it actually works as I'm used to, with BitComet opening the torrent file so that I then (usually) have a choice of which file(s) in the torrent I actually want to download.
The browser is Mozilla Firefox version 3.6.13 (on my desktop) or 3.6.3 (on my laptop)