Empty slide with prev/next buttons

Sometimes, while you click on next/previous buttons, instead of the next image, an empty div appears.
It never happens if you don't click the buttons. Also, it only happens ~30% of times, not always.
[See screenshots]

Imagine I have 3 slides: A, B and C.
A is shown, after 3 seconds, B appears, and after B, C appears. I click on "PREV" button, and instead of B, I see an empty div. Now, 3 different ways:
1) I click on next -> C appears ok
2) I click fast on prev -> Always another empty div
3) I let it 3 seconds, always the next div appears without problems.

The web is not public yet, so please, do not let google know =): universojuegos(DOT)es/drupal

I'm using drupal 7.7
Module configuration:
Display Method: cycleblock
theme layout: upright20p (with some CSS modifications)
Transition Effect: fade (i have tried different effects, same problem)
speed: 250
timeout: 3000
pause: true
next: false
Prev/Next Pager: true (pager event "click")
prev/next text: empty (i use images instead of text)
Use Pager: false
Use slide text: true

If I enable the Pager, the problem keeps appearing.
Also if I change the speed to "0", same problem.
Nothing new if I set "Pause=false".

I have firebug console enabled, but it's always clear. No errors.

So, I can't see the problem. I would appreciate some help =)
Thanks!!