I have been in our new house less than 1 year. Yesterday we had a a prolong power outage so I decided to test the generator I got for hurricane season to the house existing generator inlet. I turned off main line Breaker engaged interlock to isolate Backfeed breaker.

I turned generator while generator breaker and all other breakers are off I connected the 240/120 50 AMP cord to the outside inlet all good. I turn on AC breaker on generator all good. When i get the to house main panel and attempt to turn on the panels back feed 50 AMP breaker the AC breaker on the Generator trips off. 

House has the following:
-Duromax xp10000eh with floating neutral system connected with Generator Power Cord to 240/120( I don't know what prior owner had)
-Reliance PB50 Inlet box wired through - 4 wires(Red, White, Green, Black.) Green connects to Inlet bolt.
-Reliance MB125 Indoor 50-Amp Meter Box into (4 wires from inlet red, black, white and coppper) White and Copper connect to neutral bus on pannel, red and black connect to Q-Line Breaker.
-MainPanel with Interlock with GE 50 AMP - Q-Line 50 Amp 1 in. Double-Pole Circuit Breaker(thin).

Any ideas, at first I thought it was the generator but this generator has a floating neutral. I am not sure if there is a fault somewhere or the breaker may not be appropriate for the generator. 

Thanks in advance