Thank you all for your kind replies, tried all the suggestions ofcourse. I guess it must be my Firefox/Windows system acting strange. Just tried on a Mac, same file, actions and dropbox, no problems.
Sorry for connecting the wrong dots ;-)
I'm trying several things to fix it, chrome works again, FireFox still won't let me save a larger project over the previous one. I think I will choose to reinstall FF (it's so much faster executing CS3 games on my system). Will figure it out.
Again thanks for your time, but it was indeed not CS3 related.