I’ve managed to solve it myself after receiving some information from Victron.
My PV energy meter was only transmitting the current power being generated by the solar panels, and not the total energy generated as an “energy counter”.
“A few months ago vrm has included a rule to only give a solar forecast to vrm systems that have pv energy counter somewhere in the system.”
I modified the Python script, and it now also reads the total amount of energy generated via Modbus. Shortly afterwards, the solar energy forecast was available again.