SUMMARY
***
When you start System Settings, you can't download anything from the Internet, like for example: Icons (applies to all the new things I want to download from the Internet in System settings: Fonts, Global Appearance and all the rest). 
***


STEPS TO REPRODUCE
1. Open System Settings
2. Click Icons (for example)
3. Click Download new icons
4. Select any new icons, open them and then click upload in the upper right corner of the window.
5. The "An error has occurred" window opens. 

Error message:
"Downloading "..." failed, error: SETTINGS invalid number of concurrent streams"

OBSERVED RESULT

"Downloading "..." failed, error: SETTINGS invalid number of concurrent streams"

EXPECTED RESULT

New icons are downloaded. 

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: openSUSE Leap 15.4
(available in About System)
KDE Plasma Version: 5.24.4
KDE Frameworks Version: 5.90.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION

1. As I wrote above, it applies to all things to download from the Internet in System Settings
2. The messages I described may be a little different in English. I use the openSUSE version with Polish - I translated the messages into English myself.