Catchable fatal error in theme

Today I upgraded wordpress from 2.7 to 3.0. Unfortunate I get the error:
Catchable fatal error: Object of class WP_Error could
not be converted to string

When I go to that error (index.php line 154) I get this:
<h3 class="<?php echo $bm_boxColor[ $bm_currentColor ]; ?>"><a href="<?php echo get_category_link( $catDetails->cat_ID ); ?>"><?php echo $catDetails->cat_name ?></a></h3>

Since my PHP reading skills are basic at best, could someone please describe what this error means?

I tried turning plugins off and on again, that didn’t work.
I found one solution which said every category should have a post in it, I did that (created one post and assigned every category to it) and that didn’t work.

I think however that it does have something to do with the categories but I don’t know what.