I've made 5 or 6 succesfully flights with FSE, none of them gave me an error.
What I did: First, put fuel on FSE page, convert it to kg (2.688 kg per gallon, FSE numbers). For example, 7500kg
Then, in the Sim with XIEG menu load 7500 kg, save it
After that, start FSE client, log ing, start flight. What I noticed is that it split fuel between the three tanks (for example, I load 3500 in each wing tank, 500 kg in center tank. Start FSE client ando it splits fuel in 2300kg, 2300kg in wings and 2900kg in center tank more or less)
I enjoy my flight and get some money