Has anybody a solution, have a pleasant evening.

I am not sure, but what first pops up in my mind is:
on the level - never
at the level - always
so, I am not sure if you can use on the level, or at least I have never heard of it.

In BE the phrase "on the level" can often be heard in conversation to imply honest/true etc.

agree with en - "on the level" means valid, honest, true, etc.

but at for levels that show height, etc -
at eye level
at an advanced level