I'm having the same problem on my recently acquired 1974 europiccola. I've pumped a ton of wd40 into the threads but it's made little difference. The belt wrenches are useless, a better option (the closest I've come with a wrench) is for the vice grip type filter wrench (see this link for a photo).
It's been able to grip the flange nut but hasn't applied sharp enough force to loosen it. The problem is that constant force is not very useful on difficult to grasp nuts and bolts, a sharp impact is better to loosen them. Most people just recommend a hammer and screwdriver, give it a few whacks on each of the little ears that the element bolts on to (not too violent) and hopefully it gradually loosens.
I've had no such luck yet, it's a real pain. They design the lever machine to be so simple and straightforward to disassemble and service and then just screw it all up by doing something irritating like this.