Parse error at char 4: unexpected character: '\ufeff'"

I am running a grafana server 7.3.5 on a Ubuntu 18.04.5 system to visualize metrics from a ceph server via prometheus. I downloaded a dashboard #9550 for this purpose which works fine. Now I wanted to expand this dashboard with a new panel.
So I create a new panel and then paste a Metric expression from an already existing and working panel into the new one:
I select prometheus as data source just like in the original panel and add a panel-name. Now comes the strange behavior. I tried his say 30 times and in some cases (perhaps 3 times) a saw the data visualisation as expected and in the rest I only get a strange error from query explorer:


error:“parse error at char 4: unexpected character: ‘\ufeff’”
message:“parse error at char 4: unexpected character: ‘\ufeff’”

I always copy and paste the very same Metrics-expression into the panel. There are a few other people reporting the same issue, but I did not find any hint what this problem is all about and how to solve it? Any ideas?

Thanks Rainer