Got an error within the graph "results[iter].datetime.match(...) is null"


I’ve installed newest version of Grafana 6.6.2 with PRTG plugin (4.0.4) from Jason Lashua but if I try to build up graphs, I only get an error in the left upper corner of the graph “results[iter].datetime.match(…) is null”. What is the reason for that and how can I fix this?

Thanks a lot!