Could you enable session tracing, retry the download and look into the session log, maybe you find some useful info for troubleshooting there.
-> https://support.solarwinds.com/SuccessCenter/s/article/Change-NCM-session-trace-location
Alright, even though I think this won't lead to a solution, here's the way how to create a new device template:
In the wizard, pick your device and follow the wizard. You need to have the commands ready that you would enter in a SSH session to get the Config.
Hope that helps.
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