I have the same issue.

my 'unc' folder automatically set security without fullcontrol for SQLSERVERAGENT when I setup the publictions. I had manully granted full control to SQLSERVERAGENT. (xp_cmdshell enabled. [NT SERVICE\SQLSERVERAGENT] in sysadmin role)

I didn't find other process is in use on those folders(expired snapshots). I can manully delete the unc's subforders.

appreciate any helps...