I am running OMV5.5.9-1 on two machines(just updated).
I've seen the following phenomenon before on one of the machines, now I have it on both:
In System onformation -> Performance data (Systeminformationen -> Leistungsdaten) I do see all the graphs (except Disk I/O which seems to be broken since longer because of kernel incompatibility if I remember well?) but all the graphs are empty.
Does someone have an idea how to fix this or where I could look in order to give you more details?