Why am I getting a warning about conflicting distribution on apt update and the system becomes unresponsive after apt upgrade

Question

Whenever I do some suggested operation from the GridDB docs followed by

sudo apt update

I get a warning as below:

W: Conflicting distribution: https://www.griddb.net/apt griddb/5.3 InRelease (expected griddb/5.3 but got )

And, when I do

sudo apt upgrade

the system becomes unresponsive.

My server is Linux Ubuntu 20.04.