Error 'Fetching tags failed: undefined'

Using Grafana 8.0.6 with graphite backend, data provided by telegraf. Both Grafana and graphite are running in docker.

The issue
When modifying or creating a new Panel with graphite as my backend, as soon as I click on ‘Select metric’, I receive the message ‘Fetching tags failed: undefined’. This does not seem to affect the functioning of the dashboard, but I wonder where it’s coming from, and how to fix this message.
This message appears only once. When I close it, and click on ‘select metric’ again, it does not appear. Switching to another datasource, and then back to Graphite makes it appear again, though.

In pictures
Click on ‘select metric’

Error message in top right corner

Some more background
I might have caused this problem by manually removing some data from the graphite system, by manually removing whisper data files, as per instructions found here.
Restoring the data files did not fix the problem, though.

The question
What causes this message, and how can I get rid of it?