TLDR; I spent nearly two hours troubleshooting my broken system, because I installed a Windows spell checker for my LibreOffice.

1. Install the .oxt file for your Linux LibreOffice installation
2. Don’t realize it was for Windows only because it installed fine on Linux
3. Freeze your system completely for 15 seconds, after which it’s business as normal
4. LibreOffice works okay, so don’t notice anything else
5. Install additional spellers from Synaptic because the first one didn’t work
6. Realize Linux Mint Software Center GUI is broken and most of the flatpacks aren’t displayed
7. Perform two system resets using Timeshift, nothing changed
8. Realize the speller you installed was Windows-only, purge all LibreOffice components, problem solved, reinstall LO
9. Also realize you had to install a system package version of LibreOffice (instead of Flatpack) for the speller from Synaptic to work
10. Feel like a noob

🫣

EDIT: It happened again. I think this time I figured it out for good. I installed the spell checker through Synaptic, but either it was the wrong version or it didn’t install all the necessary packages. I found the right package in Software Center itself and installed it. Everything has been working okay, the Software Center hasn’t bugged out yet.

EDIT 2: Okay, now I’ve got it. It was a icon theme that I installed from Cinnamon Looks called FairyWren which hid half of my installed apps, and created all sorts of GUI bugs and made Software Center hang and freeze. I’m going to write to the author of the theme.