Strange output in Grafana from Nagflux + Histou

We are in the process of migrating from pnp4nagios to Nagios + Nagflux + InfluxDB + Histou + Grafana.
So far (with some exceptions which is irrelevant to this question) everything looked good until this morning.
After a restart my Grafana (Histou) graphs are not created as a “Graph-Line” but as “Points” after the restart.

If I select a "time frame " of 3 hours (instead of the default 8 hours) the graph looks fine:

Selecting any time frame >= 8h and the graph looks like the first screenshot…

Any suggestions?