### A NEW CABLE SOLVED FOR ME ###
Cable!!! Yes, believe it or not, the solution was to use Apple's Thunderbolt 3 USB-C expensive cable.
Since Jul 2023, my setup has been a windows Dell laptop on a stand to the left, a MacBook Pro M2 on a stand to the right, one Dell monitor in between them and a single microsoft mouse and keyboard combo using a USB-A receiver, all connected by a TESmart USB-C KVM.
That gives me the sweet ability to keep both laptop's screen monitoring stuff and a single central big monitor, mouse and keyboard that I can easily switch between the two computers.
This particular KVM model uses a single USB-C cable to connect to each PC/Mac and came with appropriate cables that were tested by the manufacturer, since poor cable quality is a know issue among KVM users.
Till this morning, I've installed every macOS update w/o any problem, when the macOS Sonoma 14.4 prompted to me. Immediately after the installation, mouse and keyboard stopped working on the Mac (still working fine on Windows), even thou the monitor was still there, switching as before.
I tried a bunch of different connections, including dongles and a Dell Dock. Reboot this, reset that. Connect only monitor, connect the dongle on the monitor's USB hub, use another USB-C port on the Mac... Every solution worked fine on the windows, not a single one on the Mac...
The only way to have mouse and keyboard working on the Mac (that was fine minutes before) was to connect the Receiver straight to one of it's own USB-C port, but that would brake all my setup, since I would have to physically move the USB-A receiver between the two computers every time I wanted to switch.
Full of anger and in complete regret for installing the **** update, I tried the last available thing, in total disbelief: changing the cable. Immediately after connecting the Apple Thunderbolt 3 cable, it worked, just as before. I then removed all the dongles and docks out of the way and returned back to my original config and all worked fine.
My guess is that the Sonoma 14.4 has a more rigorous implementation of the USB protocol (or whatever name) that my good, certified cable isn't able to handle anymore.