When you attempt to make your files and folders offline the below error appears:
Offline files (\\server\share): Unable to make file Filename available offline on \\server\share. Files of this type cannot be made available offline.
This error is usually caused when you try make one or more of the below files offline.
.db?
.ldb
.mdb
.mde
.mdw
.pst
.slm
If you are experiencing the above error, it is possible that the file extensions have been removed from the list of default excluded files. The above file extensions are exluded by default.
To check or re add the extensions, please follow the below:
1) Access the Policy which you wish to amend or create a new one and link as required
2) Expand Computer Configuration
2) Expand Computer Configuration
3) Expand Administrative Templates
4) Expand Network
5) Click Offline Files
6) Click Files not cached
7) Next type the following extensions to exlude: *.slm;*.mdb;*.ldb;*.mdw;*.mde;*.db?
8.) Save
For further details, see Microsoft KB252509
I would recommend you test the above solution before applying live to the whole network.