I also own a modded Canon M100 and when using EKOS and INDI I am unable to get exposures longer than ~300 secs, with the same error and same log output.
Absolute max I was able to get is 320sec, however erratic problem start at around 300sec.
The camera can definitely go longer - I am also using Cascable - a 3rd party (eg. non Canon) remote control smartphone app and I can expose for 6 or 10 minutes without any issues, same when using camera directly.
It is really great camera for astro - light, small, cheap and with very good sensor so it is really frustrating to have this issue.