The message “Unable to back up X files (next attempt within Y minutes)” is most often seen when the source computer runs Windows. This is an information message and not a warning or error.
Normally, this means that the files CrashPlan is attempting to back up are open and CrashPlan is unable to back up the file. CrashPlan will continue trying to back up the file. Once the file is closed, it will be backed up. Full information on open file support in Windows is available here.
If you are technically minded, you can view which files are not being backed up from the backup_files.log.0 file. A W at the beginning of the line indicates a warning and those files were unable to be backed up. The log directories for all platforms are listed here.
Example of a file that could not be backed up:
W 01/02/09 10:52AM 42 - C:\Users\Administrator\AppData\Local\Microsoft\Outlook\Outlook.pst
Once the file is closed and CrashPlan has backed it up, you'll see an I at the beginning of the line.
Example of a file that was successfully backed up:
I 01/02/09 11:41AM 42 - C:\Users\Administrator\AppData\Local\Microsoft\Outlook\Outlook.pst