It is permanent.
It happens only for one bucket (for the other buckets all the graphs are shown).
It is on a couchbase cluster of 3 nodes.
If I select one specific node from the upper right drop down menu, graphs are shown correctly (related to that node), while "All Server Nodes" graphs are not.
It happens both on Chrome and Firefox, on linux os.
In the logs I only have the log error I posted, it is printed every time I refresh the bucket page.
Thanks for your help!