The thing I don't understand is how come this DOES work on our linux setups

I do not think it actually works... I could not have find a code that will convert the ENV in the config string ...

I'll be happy to test it out if there's any commit available?

Please do, and feel free to PR it 😍
https://github.com/allegroai/clearml/blob/d3e986393ac8d1a1ea48302224962570ab8e6f9e/clearml/backend_api/session/session.py#L576
https://github.com/allegroai/clearml/blob/d3e986393ac8d1a1ea48302224962570ab8e6f9e/clearml/backend_api/session/session.py#L586
https://github.com/allegroai/clearml/blob/d3e986393ac8d1a1ea48302224962570ab8e6f9e/clearml/backend_api/session/session.py#L613
The same example code in these three and you are good to go
import os ... ... os.path.expandvars(ENV_HOST.get(...)).rstrip('/')