"thread is not attached to scripting runtime" in 2018.1.0f2

UPDATE: This occurs when Screen Space Reflections is selected in the Post Processing stack, regardless of the Preset or quality. On the upside, disabling SSP during animation testing is no big deal, and it plays just fine with SSP on at full-screen.

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I'm getting this error if I'm running with "Maximize on Play" UNCHECKED and the SCENE view open at the same time as the GAME view:

"thread is not attached to scripting runtime"

It pauses the Play mode in the editor, and I can't continue. The only reference I can find to this is in an old Beta thread, but the advice there concerned Unity Teams, which I am not (as far as I know?) using at all.

If I change from SCENE to, say ANIMATOR, the game runs just fine. This is great and all, but if I need to have the scene view and the game view open at the same time to inspect animations from different angles, this is a big roadblock.

Any ideas or suggestions would really be appreciated!