This is weird!  I tested the update on our DEV server ( clone of our live server) and it updated 100% fine to 2.11
Joomla 3.10.11 on PHP 8.0.27

BUT, when I then went to update on our LIVE server, I got this error:

An error has occurred.
-1 Copy file failed

I can't find any trace of this in PHP error logs, or Joomla  
Any hints about where i might be able to see what files are failing and where permissions are out. ???
(Actually they should not be any different than the permissions on our DEV server, where the update worked...)

after the failed update, the K2 blogs all work fine in front end and back, unless i click on a link to the category.. then I get this:

 Error: 1176 - Key 'idx_item' doesn't exist in table 'i'

ouch!!