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

Hello,
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:
sum(rate(ceph_osd_op_r{job="$job"}[$interval]))
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:

request:Object
url:“api/datasources/proxy/1/api/v1/query_range?query=sum%EF%BB%BF(%EF%BB%BFrate%EF%BB%BF(%EF%BB%BFceph_osd_op_r%EF%BB%BF%7B%EF%BB%BFjob%EF%BB%BF%3D%EF%BB%BF%22ceph%22%EF%BB%BF%7D%EF%BB%BF%5B30s%5D%EF%BB%BF)%EF%BB%BF)&start=1608193020&end=1608196620&step=15”
method:“GET”
hideFromInspector:false

response:Object
status:“error”
errorType:“bad_data”
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