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                        PDF reader is not loaded
As shown in the gif below, when I click "Read first in the queue", the PDF reader is not loaded. Normally it should appear on the right. What's going on?

Looks like the display mode is set to Fields. But I wonder why you don't have the button on the bottom left (should be right below Tags) to set the display mode. Has that button always been missing for you or just recently?
Also, what Anki version are you on?

I never noticed that button, so I am not sure when it went missing. My Anki version is 2.1.35
Looks like there is a conflict with the Opening the same window multiple time addon. When I have this addon enabled, the button to set the display mode disappears.
That must be the reason, those two have never been compatible as far as I know.