UPDATE:
I’ve been trying different ways to have this problem solved or work it out somehow so I can continue to use LO Impress.
I tried using safe mode to open previously-created slides. Didn’t work.
I tried resetting the user profile (via safe-mode options). Didn’t work.
I tried creating a slide and saving it in safe mode. It worked.
I tried opening and editing and saving this new slide. It worked.
Now, I can create new slides and save them normally.
But the old ones I already had, can’t be opened.
Before creating a slide in safe mode and saving it, I wasn’t able to do so in normal mode. I think this, somehow, “fixed” the problem of creating new slides… At least I can now do so. I’ll keep posting if anything happens…
Thanks for your time.