Open with Password Problem

For two straight days I have password protected a PDF to require a password to open the document.  Immediately after I set the password up I can close the file.  Then reopen it  . . .enter the password and the file opens.  The next day however when I go to open the file and the enter password prompt comes up.  I enter the password and I get the error.  Incorrect password.  It has nothing to do with caps on.  It just refuses to accept the password. 

I never have experienced this problem until ADOBE DC.  It is creating a huge problem, particularly with older PDF that have been password protected.  That is why I tested with a new PDF file.  Anyone know why this happens or how to correct it?  Thanks Doug