date_popup + theme = infinite loop
Stab in the dark, but I think your problem is probably stemming from the fact that you're building the form from within a theme function, which in turn calls another theme function, then so on and so forth. Logically theming should be after you've built the form.

Replace drupal_get_form('mkl_variousTest_form') in hook_theme() to NULL and in your page callback add the following code.

$form = drupal_get_form('mkl_variousTest_form');
return theme('formPage', array('form' =&gt; $form));