To save on disk space, OneDrive (and some other cloud storage providers) use a system where some files are only "virtually" stored on the local disk, and are materialized from the cloud storage on-demand.
When you use the "Take filesystem snapshot" option in Magnus Box, Magnus Box takes a VSS snapshot of the disk. This is a read-only snapshot.
When you back up the OneDrive directory with VSS enabled, OneDrive is not able to download files into the snapshot, because the snapshot is read-only. This causes the "Media is write protected" error message.
In this situation, your OneDrive data is not being protected by Magnus Box and is not available for restore.
You can workaround this issue by creating two Protected Items: one with VSS enabled, that excludes the OneDrive directory; and a second one with VSS disabled, that only includes the OneDrive directory.
Note that if OneDrive needs to materialize a lot of data from the cloud, then backing up the OneDrive directory may cause a lot of data to be downloaded.
A future version of Magnus Box may avoid this issue by automatically disabling VSS for the OneDrive directory.