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 your profile, go to the Compression tab, and tick the option Try and compress files that are being used.