Compression error: Cannot open zip file. (503)

This error can be caused by the following:

• The Zip file doesn't exist because it could not be created, e.g. you have no write permission to the folder in which the Zip file is being created.
• The Zip file does exist but you do not have read permissions for the Zip file.
• Another program is using the Zip file and has it locked. This stops other programs, like SyncBackSE/Pro, from reading from or writing to the Zip file. Desktop search and anti-virus programs may be temporary locking the Zip file. This FAQ article explains how you can detect which program has a file locked (note that the program may have released the lock by the time you check).
• Some users have reported this error can also occur if there is a bad sector on the drive where the zip file is located.