This was reported before.
In a couple of threads it was attributed to either a too low of an offset value or to WB_R/WB_B values.
Well, there's something else.
I am not sure how consistent and repeatable this is but I had this on many occasions.
It goes like this:

-- take a sequence of long Light exposures.
-- take bias images (select Type: Bias in Ekos) - bias images will (often) be empty, no signal whatsoever.
-- to force bias images to work, I switch back to Type: Light and take several short light images at 0.1-0.001s. Probably, just one 0.001s would be enough.
-- after that bias images are taken correctly

Windows client. SM 1.5.4 on RPI4 4Gb.

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