Calc keeps asking for a password when opening a xlsx file on a samba network with Windows server and linux mint 15 client. Other file types do not have this problem.
It sounds like it is file - specific then since nothing else wants a password.
I have used passwords in calc before to prevent other people from accessing secure files. When you go to File / Save As/ look at the bottom - is the box checked that says to save it with a password?
If yes and you don’t know the password simply save a new copy of it without that box selected. I don’t think lost passwords can be recovered - you just have to save a copy.
Hope this helps.