I noticed that when I create an instance of mod_k2_tools using the Tag  Cloud option and add a suffix to the module, the new module has a space between the class and the suffix. Other modules when you add the suffix you have class_suffix but in this case it was creating something like class _suffix.
The issue is apparently on line 15 of the tags.php template:
Where:
class="k2TagCloudBlock <?php echo $params->get('moduleclass_sfx'); ?>"
Should be:
class="k2TagCloudBlock<?php echo $params->get('moduleclass_sfx'); ?>"

The resulting class with the extra space causes the class to be seen as two separate classes instead of one. I am not sure if this is intentional as all the other templates for mod_k2_tools have the same extra space... In any case, I created an override for this and it works for me. 

HTH,

Flavia