Server Blocked After Moving Big Files

After moving a directory with subdirs and some (big) files and later deleting the files the server is now blocked with messages “Operation canceled”. The admin log contains several entries like this one

Even after manual removal of the files from the server the error messages persist. I now have two questions:

  1. how to solve the current problem (e.g. remove offending files [where?], reenable synchonization)
  2. how to avoid the problem in the future