Can't show Energy Usage chart

running OH2 on a Raspberry 3.
so I installed the rrd4j persistence service and created the rrd4j.persist file, which contains only the energy-usage item from my power outlet:

Strategies {
everyMinute : "0 * * * * ?"
everyHour	: "0 0 * * * ?"
default = everyChange
Items {

Lamp_EnergyUsage : strategy = everyMinute, restoreOnStartup

I now tried to show the chart, but neither HABpanel nor a basicUI sitemap work.
The log shows:

Chart generation failed: null

I checked the persistance service via a REST API call, the database records the information.

My basicUI sitemap was this:

sitemap test label="Test" {
    Frame {
            Chart item=Lamp_EnergyUsage refresh=10000 period=h legend=true


Anyone knows whats the problem?

If I refresh the HABpanel site I get a long error message in the log, I’ve attached it.

errorRRD4J.txt (5.0 KB)