Changed mine like LordHorst suggested, but still get an unknown error. My results correctly returns on my system, but will not on this Kata.

Edit:
Got it to work by not commenting out [Test], don't know why it works with it now

In English, y is not a vowel by default. Rule is sometimes y, so Kata creator does not include y as a vowel.

Fixed the give test so it works with the following:

Change of the name of the solution class to Kata and it works

Ran your code with my sample 1236280995 and gives an error:

Value was either too large or too small for an Int32.