Name for a fallacy, if it has a name

X is very difficult to understand; indeed, I can’t understand it.
Therefore, no one understands X.

Sometimes there are more leaps, e.g.
I don’t understand X.
Therefore, no one understands X.
Therefore, anyone who claims to understand X is lying and probably in on the conspiracy to mislead us into believing X.

Does this fallacy have a specific name? I know that its related to the Hasty Generalization and (maybe) Argument from Ignorance, but it’s not exactly either of those.

Maybe this?

Yes, thank you.
I couldn’t remember it and found it difficult to google.

It looks like it could be the fallacy of the undistributed middle?
Fallacy of the undistributed middle - Wikipedia

It is not always a fallacy. Richard Feynman said (in many places including in a lecture I heard) that he didn’t understand quantum mechanics and if he didn’t, no one did.

True, it’s an informal fallacy.