Ok, found it. My internet provider TalkTalk blocks map downloads if KidsSafe filter is on. Apparently, map server falls under one of the following categories:
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RE: Unable to download maps - server error
Android Windy Maps 2.0.0 (2000005)
Xiaomi Redmi Note 5, no SD card, maps are to be stored in internal memory.
cleared data / reistalled app - no improvement
I am located in the UK
When trying to download offline maps, process instantly ends with error message like "error in communicating server".
It started few weeks ago when I got notification that maps update is available.