Thanks, Astronerd, your input prompted me to concentrate on the possibility of power issues. It paid off:
A little bit of history: Three years ago I purchased two Raspberry Pi Model B and two 5.25V 2.4A switching power supplies from Adafruit. (One of those two power supplies has since gone missing.) The remaining power supply I use on one of the Pi's that I made into a stepper motor controller using an Adafruit HAT for my telescope. It works flawlessly. The other Pi sat in a drawer for all these years until now when I installed Astroberry. Like I said I only had one power supply so I used it to test the Astroberry. That's the problem:
That three year old 5.25V 2.4A power supply sends 5.45V to my Altair camera. I measured it using this "USB Charger Doctor" from Adafruit:
www.adafruit.com/product/1852
Current fluctuates between 80mA and 140mA.
As I described in my original post the camera will capture a few frames and then hang for 3 minutes before timing out.
Last week I purchased a second power supply from Adafruit, this one advertised as 5.25V 2.5A:
www.adafruit.com/product/1995
This one outputs:
5.31V after booting
5.26V after launching KStars
5.23V after launching INDI/Ekos
5.23V while capturing frames
With this new power supply there is no hanging!
For comparison I hooked up the camera directly to my Linux laptop and ran the same tests. No hanging and voltage regulated perfectly at 5.23V.
Thanks for your help!
PS: I would have to dig into the specs but it looks to me as if the Pi simply routes the power from the power supply directly to the USB device. I guess it assumes that the device has its own voltage regulator. Apparently the Altair camera does not regulate it. It just uses it in its raw state.