thanks for the cross-link.
We are currently in contact with the vendor of the file system driver (Callback Technologies) that apparently causes VSS to break under certain mount conditions, we will let you know once we have a proper solution.
The current workaround is to enable
Mount for all users and
Mount with Windows Mount Manager in the advanced Boxcryptor settings.
PS: I changed the title since it is a general VSS compatibility issue, not only affecting Macrium Reflect backups.