Good. No one else reported any similar problem. I'm sure the problem was not due to the upgrade.
Instructions for resetting the password are indeed complicated. I might build master password reset into WinSCP GUI eventually.

I finally got it to work. Thanks for the help.

A couple of notes:

1. I found the instructions on where to find the configuration settings were extremely confusing, to say the last. I never did locate the configuration data in the registry. None of the online examples worked for me.
   
   I would strongly recommend you rewrite those instructions for clarity.
   
   
2. My password problems appeared immediately after upgrading to version 5.21.3. I'm scared that the problems will return on the next version upgrade as well. Did you ever determine what went wrong with the 5.21.3 upgrade?
   

See https://winscp.net/eng/docs/master_password#reset

I thought perhaps changing the master password might help things. But when I tried that, I got an error saying I didn't use the correct master password. So evidently I didn't remember it correctly. Screen shot is attached.

So now the problem becomes different. If I don't know the master password any longer, how do I get around that? Should I de-install WinSCP completely, then re-install it?

Can you please post series of screenshots documenting what exactly are you doing?

No to both questions. I never see any stars or dots.

When I save the site again, it asks for the master password. WinSCP accepts it and doesn't give any indication of an error. But when I restart WinSCP, it asks for a password, but only connects to the remote host if I give it the host password. If I enter the master password, it says Access denied.

Is there some configuration file I could delete or modify to make this work? I'm getting closer to abandoning WinSCP if it won't work as advertised.

After you save the site, are there stars/dots in the Password box on the Login dialog?
And are they there after you restarted WinSCP afterwards?

OK, I opened the Save Session as Site dialog, and checked the Save password (protected by master password), then clicked OK. Then, on the Master Password dialog, I entered the current master password and clicked OK.

I then closed and restarted WinSCP. The logon dialog that opened looked the same as before, but I assumed the password it now wanted was the saved master password. So, I entered the master password and clicked OK. Then I got the Access denied error, same as before.

So I tried entering the site password instead. And, as before, it logged me into the site and I could could access the remote folder.

What am I doing wrong?

I was referring to the "session" password. Not to the "master" password. You may need to save the session password again.

No, there aren't any dots in the password field. If that indicates the master password has been lost, how do I create a new one?

I went Options > Preferences> Security, but it doesn't offer a way to create a new master password, only to change the current master password (which doesn't work). So I'm stuck.

How do I fix this?

From your description it looks like the password is no longer saved in your site. Are there stars/dots in the Password box on the Login dialog? Wasn't the password somehow lost when you were dealing with the previous bug?

I just installed version 5.21.3 on my Windows 10 machine. I no longer get the error message about master passwords, but WinSCP never asks for the master password.

If I enter the master password to get access to a host, it says bad authentication. I have to enter the host password to get access.

I double checked, and I do have the Use Master Password box checked in Preferences.

This problem is 100% reproducible.

What am I doing wrong?