Problems with screen overlay and permissions

I am being blocked from using many of my apps. When I try to launch a popup says 'screen overlay detected' and there is a link to Settings to disable it so that relevant permissions can be granted. I then get just about every error I can think of, depending on which app it is:

1. the app is not shown in the list in Settings
2. if the app is listed it may show the overlay permission is already switched off
3. if the permission is shown as on, it may not be possible to switch it off
4. if it...

Problems with screen overlay and permissions

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