Thank you for the warm welcome and your reply @fred1m 
. The board information and FreedomBox version are at the bottom of my previous message (although I have now upgraded to version 21.2 but the problem remains, rebooting doesn’t help).
The performance page was showing the following:
| Service pmcd.service is running | passed |
|---|---|
| Service pmie.service is running | failed |
| Service pmlogger.service is running | passed |
| Service pmproxy.service is running | failed |
After restarting these two with a sudo systemctl restart pmie.service pmproxy.service the issue is still there so I definitely hope munin will be included in the future.