"the reason therefor" vs. "the reason thereof"

[John]

Dear all:

Which of the following two sentences sounds better and clearer?

1. If X failed to prove that a formula in Y is valid, the reason thereof
was not visible.

2. If X failed to prove that a formula in Y is valid, the reason
therefor was not visible.

The intended meaning is the following:

If
the computer program called X failed to derive a proof
of a formula that was written in formalism called Y,
then
it was unclear why the tool failed to derive the proof (maybe, no
proof exists, or the program is not powerful enough).

Any hints?

Thanks in advance
John