Raspberrypi 4 - Mouse & Keyboard not working with release 10.0.1

That's a bit weird. Is there anything else connected to the RPi4 besides the keyb/mouse combo? Do things work with only one of them? Is it the same with the USB2 and USB3 ports? Do other USB devices work, such as a USB stick with data/files?

One solution could be to use a different SDcard (or USB stick, as USB booting is supported by now), and install a fresh LibreELEC image onto it. If the matter still happens, then I'm leaning towards a hardware failure of some sort (the USB controller), although that is a rare thing with RPi devices.

A temporary fix is probably using a smartphone and control Kodi via the Kore app (Android, iOS).

If You are using two USB dongles (one for KBD, one for mouse or something else) You could try "moving out" the KBD dongle with UB extension cord.

My old K400i dongle didn't like to sit next to my remotes (Rii i7 mini) dongle but with an 30cm (12") extension cord on one of them it works flawlessly.