@David-TP 

Done that in Deco app, with screenshot attached. 

As it has been 24 hours since my original post and nobody else joined complaining about the same issue, it is not widespread. Perhaps, regional, or even might be just something with app on my smartphone only.

  @Alexandre. 

Status update: problem solved.

I have not reinstalled Deco app.

I have received email from TP-Link support and they noticed one of Deco nodes in my mesh, nicknamed "Office," appears offline according to Deco app logs. They suggested to power it off and on. After I did that, "Update Deco" functionality was restored.

I would like to point that, during the ongoing issue with “Update Deco” failures, Deco app reported “Office” Deco node as online. More than that, I had multiple wireless clients connected to it and also all-in-one network printer connected by Ethernet cable. People in my household and myself use that printer very often, I can guarantee “Office” Deco was capable of passing network traffic and otherwise looked healthy.

Which means, by just checking state of Deco mesh in Deco app, it would not have been possible to figure if anything is wrong. 
 

Next time issue like this happens, and Deco app reports all Deco mesh nodes as online and healthy, the recommendation would be to either send logs through Deco app for TP-Link support to check if anything is wrong, or to just power off all Deco nodes and power them back on.