Could not back up sites directory for drupal

QAll attempts to run Clone or Migrate task in Aegir fail with mysterious error “Could not back up sites directory for drupal.” Why it affects only one of my sites?

APlease move away the .sass-cache sub-directory, if exists, from theme directory in the affected site. You may need to do it via SSH, since dot directories are by default hidden in FTPS and SFTP, unless you will make them visible in your FTPS or SFTP client. It is normally not needed to remove dot directories. The .sass-cache can sometimes cause problems, especially if there were changes and you can’t wait for the nightly permissions fix procedure. Incorrect permissions inside .sass-cache can cause problems for tar, which results with non-clean output and breaks the task. The backup sub-task previously would continue anyway, which often resulted with migration broken too late. Now Aegir better detects such issues and fails early. The recommended solution is to avoid site specific space for your code and themes, and always use /sites/all/ space, if possible. For more details please read The best recipes for disaster, especially Recipe #4 there.

»What if you don’t have any .sass-cache directory? There can be still some other dot directories, like .git or even some tmp files in the site’s files directory which break the tar archive and subsequently also the Drush task. It is a good idea to either remove them, or move elsewhere before running Migration in Aegir again.