I’m checking with SHN support about this but my memory has the devices3.xml changes happening at the 1.54 release which would mean there is no need to convert when starting with 1.54.
Can you expand on the symptoms you are seeing? Any error messages that are displayed. What device(s) can you not add now, what type of device is it. Is this a new device that was recently installed? This may be a powerline communications problem rather than an xml compatibility issue. Are you using Access Points or Signal RF devices to couple the powerline phases.
SHN confirms you do not need xml conversion. The 1.54 release you started with had the new format. Unless something happened to the devices3.xml file during the attempt to convert, your devices3.xml file should be fine.