I’m still not certain of the train of events.
You flashed a new SD card with the 2021 image, then you manually tried to upgrade it?
Between putting the new SD card in, did you power it and leave it for a very long time to allow it to automatically update itself, or did you immediately (or very quickly) jump in and try to do things?
If the latter, I think you might have halted the automatic update and left the system in an unknown state. In that case, flash the SD card again, put it in, power it up and leave it for a long time (how long, I’m not sure, but it’s much more than a few minutes, or even a few tens of minutes.)
If you are re-flashing the SD card, put a plain text file (it can be empty) called “ssh” in the /root directory before you install the card in the Pi - this will allow SSH access while it is updating and afterwards, which is always useful.
(Some details here: EmonPi and SSH and I find a pair of key files extremely useful - without a password set, they authorise the machine you’re using to access the Pi, not the user.)
If you’ve still got a problem, do you have an FTDI - USB programmer ( Development Boards - Programmers - Shop | OpenEnergyMonitor from The Shop, or its predecessor)? That would allow you to run the “Emon” front end separately from the Pi and establish beyond doubt whether the data was being generated by the front end.