We had the exact problem on our sob with a 5500 generator.
I'm usually pretty good at finding/fixing problems like this
I have to admit
I accidentally found the trouble.
The 2 breakers on the 5500 are covered by a "thick" cover with 2 holes for the breaker switch to stick thru.
While checking the oil I accidentally bumped that cover, generator
was not running
click, wonder what was that, I saw the tripped breaker.
I'll shorten the story here,
the hole for the breaker was .0? TOO SMALL
.
I drilled the hole a tiny amount, test OK.
Give it a try it's free.
Good luck and keep updating