Solved!
The problem was with an existing class of my site template, that had the same name of the class used for the download button.
To solve the problem needs to change the class name. I made the following changes, inserting new class name "downloading":
in components\com_phocadownload\views\category\tmpl\default.php, row 149:
[echo '<td><div class="downloading"><div>';]
in components\com_phocadownload\assets\phocadownload-grey.css, rows 88-97-102:
[#phoca-dl-category-box .downloading {]
[#phoca-dl-category-box .downloading div {]
[#phoca-dl-category-box .downloading div a {]
in components\com_phocadownload\assets\phocadownload-blue.css, rows 88-97-102:
[#phoca-dl-category-box .downloading {]
[#phoca-dl-category-box .downloading div {]
[#phoca-dl-category-box .downloading div a {]
in components\com_phocadownload\assets\phocadownload-red.css, rows 89-98-103:
[#phoca-dl-category-box .downloading {]
[#phoca-dl-category-box .downloading div {]
[#phoca-dl-category-box .downloading div a {]
Enjoy this excellent component!