I have a Multiplus ii 3000va. I borrowed a Honda 1kva to charge our batteries, and it worked fine. I bought a yamaha 1kva, everytime I put it into Charge mode or inverter with no loads the generator goes into overload and cuts the 240v supply off. I have tried turning off the Ups, assist current boost, turning on Weak ac, Dynamic current limiter. And even tried turning off Ground relay, disconnected earth input, disconnected earth output. Nothing seems to work. Any help would be appreciated.
Your generator is too small to pick up the initial load of the multiplus. See the above for recommended sizing.
Whilst it is possible to use smaller generators, inverter generators are not recommended, due to their (lack of ) surge capability.