I'm going to try a different OS this weekend. But in the meantime...

Are you able to use this camera without Mirror Lock?
Disabling Mirror Lock is just a matter or setting the Mirror Lock time to 0 on Ekos and set it to be "off" on the camera menu?

Apart from disabling "force bulb" and picking a predefined exposure time, what else do you ensure is set in order to take short exposures?

Thank you.

Yes I tested it with and without mirror lock. Set it to 0 to disable it as you already figured it out. With 600D, I actually don't do any changes to take short or long exposures, it works equally well on both. I take take from 1/8000s to 300s without touching any settings.

I see, it's indeed boiling down to a OS suspicion. This weekend I'm going to "dd" the current SD card of the Pi to backup, and then try it with Ubuntu. Do you think that the client OS will matter? Because my laptop is also running Manjaro.

Thank you.

I got it! Turns out this whole time I was using the "Canon DSLR" CCD driver, I changed it to "GPhoto CCD" and everything is working just fine. All kinds of exposures.

I don't know why, but it wasn't clear to me I should't use a Canon DSLR driver if I have a Canon DSLR. lol

Anyway, I'm sorry for all the "back and forward" I created and also for my drastic thoughts.
I'm really happy I have this working now.

Thank you all that put efforts into thinking about this.
Thank you knro for all the attention.

Canon driver uses also libgphoto2 but it is slightly different in the way it queries and connects to the camera in order to provide a fuller support. But they're 99% identical otherwise.

Interesting. Using the GPhoto2 made me able to do all sorts of configuration combination with practically no issue whatsoever. Using the Canon one however, as I earlier reported, was somewhat difficult.

In fact:

I know this is an old thread, but I was having an issue that sounds very similar to this with my DSLR (Nikon D3400) and wanted to post the solution in case others found this thread.
Almost every other exposure would fail to download, but only if they were long. Short, 1s exposures worked fine, but if I tried a 3 or 5 minute exposure it would fail pretty much every other time. Well, it turns out that long-exposure noise reduction was enabled in the driver ("longexpnr" in the capturesettings tab...which I never enabled). This caused the camera to take another, equal length exposure to subtract the dark frame from the exposure that it just took. The original exposure can't be downloaded until the dark exposure is complete.

Thanks! This was added to to the documentation already by I just added it under Nikon as well.

Hello,
might be an old one, but this error message seams to lead to so many different individual reasons:
In my case (Canon EOS 5D Mark3) it was the setting (in the German Manual it's on page 146): "Rauschreduzierung bei Langzeitbelichtung" (Noise reduction on long exposures), which is a setting that is, if enabled, is applied to exposures longer then 60s. If it's turned on, then INDI can't download the images.
It took me 2 years to find that out... but it was actually this thread that led me to go after settings concerning long exposures...
To save some similar troubles to other DSLR users, maybe there should (if not already and I am simply blind) be some notes in the driver documentation, that to prevent that errors, any DSLRs setting, that does some internal image processing on long exposures should be turned off, as this might break the connectivity to INDI.

regards,
Felix