Could you please tell me what the reason that Ondemand CT scan files (DCM) are not readable by a slicer?

[lassoan (Andras Lasso)]

Ondemand scanners generate invalid DICOM files. See details in this topic:

PatientName and rus language - trouble

The DICOM files have multiple sever DICOM non-conformities. The most serious are: CharacterSet is not saved in the file. It seems that the file uses Windows-1251 code page, which is not even a valid DICOM character set. XRayTubeCurrent field is stored incorrectly (it must be an integer value, the value is 4.1 - see David Clunie's Blog: The DICOM Exposure attribute fiasco) There are also some incorrectly generated UIDs. I would recommend to contact your device manufacturer and report these …

Please report the issues to the manufacturer. Until the manufacturer fixes its software you can use the workaround described in that topic.