The Darktable repository is currently offline:
https://software.opensuse.org/download.html?project=graphics:darktable:stable&package=darktable
This causes the app Software Sources to “Refresh Software Cache” for minutes, only to report an error, after you send the report, it is still loading and you have to force close the loading screen and Software Sources.
Running sudo apt update clearly shows that one repository is offline.
Software Sources should not hang indefinitely when a server is offline right?
This is on 20.10.