I'm having this same problem, were you able to find resolution?
I've tried the admin account on a brand new app, on a brand new controller so shouldn't be any permissions oddities.
We're working through a migration from on-prem, and I've successfully been able to export/import healthrules and dashboards. Just moved onto detection rules and getting this 500 error.
To simplify things I tried the most basic export with curl based on the doc, that worked fine. Then tried importing the output of that export again just using curl based on the doc and I consistently get a 500 Internal server error.
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