Manually delete the folder. The problem appears to be that the OS has done something to make the folder ‘edited’, such as indexing files, and now FMP cannot delete it. I could be wrong but once you delete it the problem should go away.
Also, if its happening on other backups I’d make sure file indexing and virus scanning are turned off or at least set to ignore the FMS folders.
If this is your crash plan folder system could be crash plan is editing the files/folders and causing this problem.
Thank you for your response. It's possible that I've set the CrashPlan backup frequency too short and this makes FileMaker Server to be unable to delete the folder because CrashPlan has taken control of the folder in order to upload it.
Merry Crhistmas, hope to see you next year