Thank you for your response.
I'd like to report that the solution from @Igor Kurzin resolved the problem. Thank you very much for that.
To answer your questions:
A. Yes, they have worked fine previously before. This is a new issue since 3 days ago or so.
B. No. But, as a test, I added Steam as a trusted app (when trying to add Signal, KIS would crash, more info below) but it didn't solve the problem, even when I checked all the checkboxes in the Trusted Applications window.
Regarding KIS' crash when adding Signal as a trusted app, it crashes when clicking the OK button the second time (the first click does nothing) after selecting the application. However this is also resolved by the solution from @Igor Kurzin. I have sent the report offered by KIS when relaunching after crash.