edalbeci

I had the same exact problem.
For me, here was the thing though: it had nothing to do with ‘categories’, but rather the functions I placed within ‘while (have_posts…’

1. You have to disable all plugins.

For some reason, no errors were returned when plugins were activated.

2. There are two possibilities:

– categories now work

This seems to be the popular consensus – that disabling plugins would fix the problem. If all of your functions are native, this possibility should be the one.
If not, try switching to Twenty Eleven:
    -if it works, something is wrong with your theme (redownload/reinstall theme)
    – if it doesn’t work, something was wrong with your installation of wordpress (and possibly your theme too – redownload/reinstall wordpress/repeat)

– error(s) pop up

My errors actually denoted undefined functions in succession, which was quite easy to fix. Very simply, I modified the ‘functions.php’ of wordpress prior to updating, so any customized functions were obviously deleted.

This might help. For me, I had to add function capabilities to the theme – like this:

add_theme_support( 'post-thumbnails' );

Then, I had to (re)define custom functions – amongst them:

fetch_first_img () {
$files = get_children('post_parent='.get_the_ID().'&post_type=attachment&post_mime_type=image');
if($files) :
$keys = array_reverse(array_keys($files));
$j=0;
$num = $keys[$j];
$image=wp_get_attachment_image($num, 'large', false);
$imagepieces = explode('"', $image);
$imagepath = $imagepieces[1];
$thumb=wp_get_attachment_thumb_url($num);
echo '<img src='.$thumb.' class="thumbnail" />';
endif;
}

I hope this helps. =)