I think #2 would have more likely been:
No es un problema serio.
The Spanish word for problem, problema, is masculine even though it ends in an 'a'. I've never seen the word 'seriosa' in Spanish. I'm not fluent enough to be certain it is not a legitimate word. The word I've seen for 'serious' is 'serio'
But yes, I know it was a joke. And the joke might not have worked as well with a grammatical phrase.
A poll taken by the Fla. Governor's office on whether illegal immigration is a serious problem:
21% answered that it is a serious problem
79% answered "no es una problema seriosa".

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