Help! Spurious password protection...

[Mark Smith]

I'm working on a stack. I've had it open in 2.9 beta 5 and 2.8.1  
studio on OS X. I just went to edit it, and it's telling me that the  
stack is password protected, and asks me for the password. I have  
certainly not protected it this way, (I've never password protected  
any stack, I'm not sure I'd know how), so this protection is  
spurious. I've tried entering the various passwords that I use for  
other things, but no joy.

Any ideas how I can deal with this?

Best,

Mark