Encountered a similar problem today. Did not see my fix listed here, so I thought I'd share.
Root could not erase a file.
I did my research. Turns out there's something called an immutable bit.
# lsattr /path/file
----i-------- /path/file
#
This bit being configured prevents even root from modifying/removing it.
To remove this I did:
# chattr -i /path/file
After that I could rm the file.
In reverse, it's a neat trick to know if you have something you want to keep from being gone.
:)