They generally don't announce updates too far in advance.
What exactly is happening? How are you trying to open the pdf? Can you give us a screen shot of any error or messages you get when you try and use the password? Is the password protected pdf opening in Reader or during the Apple update, did Apple set your default pdf viewer back to Mac Preview?
I'm just asking these because we haven't really seen any Apple update related problems yet so at this point, it may or may not be a Reader issue.
It's our paystubs and this same thing happened a couple of updates ago, but was fixed. It either says the file is corrupt or it will repeatedly put the password field back on the screen, not opening the document. This is on an iPhone 6s plus with the most recent apple update. I've restarted and that didn't help.
Thank you for reporting the issue. Please confirm if you are providing the correct password. You can try this by opening the file in any other environment. Once this is ruled out that the password is correct, please confirm if password contained any special characters.
Have you protected the file using password protection through Acrobat ?
Please mention how you protected the file and the os version where you see the problem.