Actually it worked on my localhost as well, which is odd, since the problem appeared after I migrated to my server. I can’t tell you exact steps, since I was able to save three widgets before the fourth one refused to save the setting.
Actually, I just came to think of that I did originally change the setting on the first three widgets as well to no css, then had to go back and change and save that setting again – but for those three it worked. I’ll write again if I notice any pattern that could cause this.