Moved from ZBXNEXT-6688 (25):
vfs.file.get and vfs.file.owner refuse to monitor files with unknown user or group.
zabbix_get shows an error like "ZBX_NOTSUPPORTED: Cannot obtain user name.".
On Unix it can happen when an administrator deletes a user but forgets to reassign files to other user.
I think it can be improved by showing numerical user/group IDs instead of reporting an error, like 'ls' utility does: