Can't exit lockscreen when secondary monitor is attached

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I have a 7,1 iMac which has a miniDVI video output from the Radeon HD 2600 PRO Graphics card. This output seems to work fine as I get the Catalina wallpaper showing and can move my mouse over it. However I am unable to unlock the screen when this second monitor is attached. I tried with two different monitors with different native resolutions. But after I enter my password and hit enter, the loading disk spins, the screen goes black, and I end up back on the lockscreen again.​

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Thing's I've tried already:​

- rebooting​

- unplugging the monitor, logging in, and plugging the monitor in. However this locks the computer​

- interestingly, the second monitor does work fine on MacOS El Capitan and Ubuntu​