I’m getting exactly the same error, using PostgreSQL datasource.
I first used a Reduce expression for each query to aggregate values in the selected time range into a single value. Although it seems not correctly working as I get a single value for a specific label (and NaN for all the others).
Then I removed that as it is not needed for rules using numeric data).
It was working at the beginning, but after few minutes I started getting this error, and now I’m stuck.
Anyone from Grafana can shed light on this?