What is unknown is if God really is omnipotent. We don't even know for sure that God exists. But if God exists and God is omnipotent, then nothing is impossible for God.
We know that if those two conditions are met, then there is no such thing as "an act that is impossible for God".
What we do not know, cannot know, is the veracity of those two premises. thus, we can never claim that we have produced a sound logical argument about this topic. We can very much state we have produced a valid one, though.
In order to produce a sound logical argument, our logic must be valid and our premises must be true. Validity refers to the structure of an argument, soundness refers to the "trueness" of an argument.
Illogical refers to flawed reasoning. Flawed reasoning is actually a flawed structure. Fallacies refer to illogical arguments.
We can create a logical argument that is valid, but unsound.
In order to claim that a premise is true, it must be universally true.
"Some dogs have tails" is a true premise because it is universally true because of the fact is that "no dogs have tails" is false. Some dogs have tails is universally true even if in reality all dogs had tails because "some" is encompassed by "all". For "Some dogs have tails" to be false, "No dogs have tails" would have to be true.
Conversely, in reality not all dogs have tails for various reasons. Because that is true, "all dogs have tails is false."
If all we never encountered in our lives was dogs with tails, but we knew that other dogs existed that we never encountered, we might say "All dogs have tails". If that was in reality, where we know that not all dogs have tails, it is clearly a false premise. Even though the perspective of the creator of said premise is based on his knowledge, the premise is indeed false regardless of that creator's perspective.
Conversely, if said premise were created by a person in a hypothetical reality where all dogs did have tails, but the argument was presented by a person who only had a limited experience of all the dogs in that reality, the argument is still flawed. Such a person cannot make the statement "All dogs have tails" with honesty. Regardless of the trueness of the premise, the argument is invalid because the creator of said premise did not limit the premise to "All dogs I have encountered have tails"
It's the fallacy of converse accident.