Got 'm! I analysed the dng and like other files that hang the scan to catalog process, this dng has metadata issues.
$ exiftool -a -validate -warning gms_20160514__0058_DxO.dng
Validate : 4 Warnings (all minor)
Warning : [minor] Possibly incorrect maker notes offsets (fix by 26004?)
Warning : [minor] Undefined value for MakerNotes:InternalNDFilter
Warning : [minor] Undefined value for MakerNotes:ClearRetouchValue
Warning : [minor] Suspicious MakerNotes offset for DataDump
These are all minor (according to exiftool), but they have garbled the metadata (of the makernotes). For instance, the serial number is now garbage bytes instead of a string (probably it should have been “(F98) 2014:04:22 no. 0019”, which will be displayed by PM as F98201404220019).
I had exiftool correct the makernote offsets, and indeed the serial number now is read correctly (and so is a lot of other data from the makernotes).
Not sure what caused the metadata problem, but it is not unlikely that it is actually caused by DXO's software. Heck, even Adobe screws this up sometimes.
(but then again, maintaining proprietary makernotes metadata is not a straightforward thing).