I run the old sen2cor version (2.0.6) because reasons and get the same error you posted above ("…being used by another process").
My solution is to restart the L2A_process again, it will continue from where it stopped. I’ve put the command in a while loop and just restart it until there’s the same number of OPER and USER granules.
Pretty brute solution but I can’t find any other way to get sen2cor to work.