You could try forcing the required modules to be loaded from the initial ramdisk as it sounds that autodetection is failing for some reason.
On your server, as root, enter the following two commands:
ucr set kernel/modules="$(ucr get kernel/modules);usbhid"
update-initramfs -k all -u
The first one adds the "usbhid" kernel module to /etc/modules and the second re-builds all configured initial ramdisks.
Please post the full output of the second command here.
If the second command completes without an error then you can try booting with the 4.1 kernel again.