It's possible that you're affected by this InDesign bug:
https://indesign.uservoice.com/forums/601180-adobe-indesign-bugs/suggestions/50005143-typing-letter-in-paragraph-character-style-shortcu
You might have inadvertently typed "x" into the Shortcut field of a style (Paragraph, Character, or Object). Due to the bug, the letter will be invisible but yet assigned as the shortcut for the style. Thus every time you press "x", the style is being applied. You'll need to delete this invisible letter (see the bug description above for solutions).
It's a longstanding bug that keeps catching quite a few unsuspecting users by surprise (although I'm not exactly sure if that's indeed the cause for your particular issue).

Hi Leo,
Thank you for this! I have tried the proposed solution and it stopped the "colour change to red" when pressing "x" but still did not give me usable lower case letter.
I have however managed to resolve the issue by going into both Paragraph Style Options as well as Character Style Options and resetting all used pharagraph and styles to base (Screenshot below).

That was able to resolve the issue for me and finally have usable letter.
Thank you for the help!