Occasional error shrinking transaction log files
There is NO NEED to run shrinkfile command after taking transaction log backup. What is the need of shrinking a log file which would eventually grow again? This is a blunder in my opinion considering the fact that you are doing it every day. 

Whenever the log file grows, transactions have to wait for this event to complete so SQL server becomes still until this growth happens and considering the fact that instant file initialization is not there for log files it takes time for these growth events to complete, in turn hampering performance.

Regarding error message 


  Backup, file manipulation operations (such as ALTER DATABASE ADD FILE) and encryption changes on a database must be serialized. Reissue the statement after the current backup or file manipulation operation is completed


This means that you were trying to run dbcc shrinfile command when some backup operation was running on the database. It can be any backup full,differential and transaction log. All this message is saying that please try running shrikfile command when backup operation has completed. Now you have to find what backup was running at that time. See errrlog or msdb job history you could find more details.

If you refer to BOL


  Operations that cannot run during a database or transaction log backup
  include:
  
  • File management operations such as the ALTER DATABASE
  statement with either the ADD FILE or REMOVE FILE options.
  
  • Shrink database or shrink file operations. This includes auto-shrink
  operations.