Ran chmod 764 /usr folder
Try booting from an usb Ubuntu, mount the partition and chmod /usr to 755 for a start. Then, you will need to figure out case by case once something fails.
Good luck.
Extra info: in an almost fresh 20.04 desktop install (on a virtual machine I have)
find /usr | wc -l
gives about 177K files :-)
If you put in a bash shell file...
find /usr | while read aa; do echo $aa; stat -c "%a %n" $aa; done
you will get a list of current file perms, and you could redirect that to a file as a safe measure.
regards, JP.