Compression error: ... an error prevented a file from being processed. (527)

This error is given when a file cannot be copied because it is being used by another program. SyncBackSE/Pro can copy open/locked files, but only if certain requirements are met:

If you do not meet those requirements then you must close the program that has the file open. The following article contains details on discovering which program is using the file:

You may be able to avoid this error: modify the profile > Copy/Delete > tick the option 'When copying files, stop other processes from modifying them during the copy'.