Question: After swapping my language and input options about (UK layout and German layout) to support the two keyboards I have, a certain key will stop working partially depending on the layout.

If I swap my input method to UK, the apostrophe/at-sign key will stop working. Windows 10 will make a tone sound when pressed twice, and if held it will input many of that key, as should be the case.

If I swap the the DE layout, the would-be apostrophe key works fine, but the colon/semicolon key (Ö on the German keyboard) stops working unless held.

I'm certain it's not a hardware issue, it must be to do with Windows 10.  

What's going on and how do I fix it?

Answer: 
I solved this by removing a mistakenly added 'change layout' shortcut, to the grave (`) key, oddly affecting the apostrophe and ö keys on the other layouts.

To fix this:

* Context-menu/Right-mouse on the Language Bar on the Windows taskbar
-> Settings/Einstellungen
* In the third section, "Change the Input Method"/"Wechseln der ingabemethode", open the blue link at the bottom, "Change the shortcut keys for the Language Bar"/"Abkürzungstasten der Sprachenleiste ändern"
* From there,
remove the setting for "Change Input Language"/"Zwischen
Eingabesprachen" from whatever it is (mine was Graviszeichen (`)) to
None.
Ditto here. I had exactly the same problem (Windows 10 home, version 1909) and, as with Local Crab Enthusiast, it occurred after I tweaked the custom keyboard shortcut to change the language (in my case so I could switch between a German and a Danish keyboard layout).

There's a good description of it (with illustrations) [here](https://www.addictivetips.com/windows-tips/keyboard-shortcut-to-change-the-language-on-windows-10/#:%7E:text=Go%20to%20the%20Advanced%20Key%20Settings%20tab%20and,keyboard%20shortcut%20to%20switch%20to%20a%20specific%20language.).

Now that I've made sure that “Switch Input Language” is set to “Not assigned,” the "Ö" key is working perfectly again. It's such a relief to find out that my hardware is fine and it's just a Microsoft software glitch.

Thanks for your help, everyone!