Solution for those who have a problem with displaying wrong post from incorrect categories (not the same which currently displayed post belongs):
Modify file upprev.php, line #72, which look like this:
if (is_single()) {
and replace it with:
if (is_single() && !isset($upprev_currentPostID)) {
The modified file can be also downloaded from: http://gkrzyminski.pl/support/upprev-current-id.zip
