I may have 'a little' progress with this.

I've checked the gphoto2 version in the Linux Mint repo
gphoto2/bionic,now 2.5.15-2 amd64 [installed]

I know it's not the latest version available, but to look at it.
I've installed Entangle to test the Nikon tethering, it works fine with entangle and the shutter will remotely release and take a frame.

Back into Kstars and, in the INDI Control Panel (Main Tab) it lists the settings for manual release including exposure times. For instance 0.0002s, 0.0003s, 0.0004s etc. It also has a setting to 'Force Bulb' which I've turned off.

In EKOS, I've selected an exposure time of 0.0002s and added 10 exposures with a delay of 1 second. Eureka!! It works.

That said, it only works with exposures below 1 second. Anything above 1 second fails with the above error and so, it seems that, when Ekos appends an extra couple of 0's to the exposure time the camera isn't able to process the shutter release.

What I mean by appending extra 0's is, the INDI shows 1.0000 whereas EKOS shows 1.000000

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