Space bar function under Monterey doesn't do the right thing in Excel nor PowerPoint anymore...

Out of the blue, like it seems, the space bar does't allow making spaces between characters in Excel nor PowerPoint anymore, but still does so in Word, as well as in this message, obviously :-) In vain so far, I tried to (dis)able commands one finds under preferences -> keyboard (or mouse) --> input etc, like Apple advises here: https://discussions.apple.com/thread/251573108. Sending a random .xlsx file (via Apple mail) to my Powerbook and vice versa, the space bar does what is should do by default. It looks like I work with similar settings on Bothe my Mac mini (late 2016) and PB. Anyone out there getting the better brainwave?

[Image Edited by Moderator to Remove Serial Number]