I have never heard of this idiom. Here are the English idioms about "ground" a happy hunting ground be thin on the ground break ground break new ground common ground cover a lot of ground cover ground cut the ground from under / feet cut the ground out from under drive into the ground fall on stony ground feet on the ground from the ground up gain ground gain ground on get in on the ground floor get off the ground give ground go to ground ground in ground on grounded in fact grounds for have ear to the ground hit the ground running hold ground in on the ground floor keep an ear to the ground keep an/ ear to the ground keep feet on the ground level to the ground lose ground lose ground to middle ground on dangerous ground on moral grounds on shaky ground one's old stamping ground prepare the ground prepare the ground for raze to the ground riveted to the ground run into the ground run to ground shift ground stamping ground stand ground suit down to the ground the high ground the middle ground wish the ground would swallow up worship the ground walks on

That's a brilliant list posted by Jura. The idiom "trips on the flattest of grounds" is rare but you would hear it in England. It means that someone is liable to "trip" even when the "ground is flat" and there are, therefore, no obstacles in their path / things to trip over / mountains to climb up. Someone who "trips on the flattest of grounds", therefore, makes mistakes even when things are easy. They make mistakes when there is no good reason to make a mistake. Hope this helps! Take care and good luck!