It may be difficult problem. Satcom receivers are often more sensitive to interference than other receivers.
I would advise the following steps:
1. First if possible, try another generator. Have the generator supplier repair it or serviced.
2. Check the generator metal cover for loose parts. Check the power cable connectors and cable itself. It should be screened or grounded at the generator as well as at the station.
3. Check satcom station grounding and tightness on all covers. Check receiver cabling. Check the antenna, mainly its reflector.
4. Try to use a shield to protect your antenna from possible interference: for ~260 MHz, the shield can be made of chicken-wire mesh. Only find the mesh with welded joints. Try to place the interfering generator "behind" the antenna which must point to a satellite.