Can you check the files in
/etc/apt/sources.list.d? Do they look fine to you, especially: do they contain a lot of entries with “unmaintained” in the path? On my 4.3 installation (without additional apps, without the Cool Solutions repo) the file
15_ucs-online-version.list contains 78 lines,
20_ucs-online-component.list contains 14 lines.
You should also try
ucr commit /etc/apt/sources.list.d/* followed by