Can no longer type quotation marks in IDLE3.5. Pycharm (2018.3) and Netbeans, why?

Question

I am struggling with a weird situation. For a week or so quotation marks (', " but also ^) no longer appear when I type them in IDLE 3.5 (I get ¨ and ´ instead), Pycharm or Netbeans ( in the latter 2 I get just a space instead).

I run Linux Mint Mate 18.3 with a workman keyboard layout. In all other applications (Gedit, Vim, Eclipse, Anaconda, LibreOffice, terminal, Firefox, Chrome,...) nothing has changed and everything works just fine. The problem dates from before the last update I ran a couple of days ago (in the hope it would solve the problem) and no new hard- or software was added to my system.

Anyone out there any idea why this is happening and why it only seems to affect these 3 programs? Or suggestions on how to solve it?

So far changing keyboard layout, removing and reinstalling the affected software have not made a difference. I also have not found any other information related to this issue, which I find strange.

Thank you in advance.