I normally only get that error message if:
a) You don't have access rights to the directory.
b) Something else has the file open by another process (Alibre!) or on-access virus scanner.
c) File open in windows explorer quick view pane.
d) The file on disk is read only.
e) If its on a NAS, the RAID syncing can sometime cause access issues if it's already trying to mirror the file to another drive.
As it's constantly there, I suspect either a, b or d!
Saying that, Alibre should know exactly why it cant access the file (from the windows error code returned by the fileopen command) and either ...
a) present you with an exact reason why it cant access the file, with a few suggestions of things to check,
b) graciously retry a few times, then a)
In code, there are ways to determine which process has the file open (i.e. file handle -> process lookup), which may also help in solving the issue for the user.