Oh, I didn’t change anything else but switch to Use Category URL Titles In Links. The site has been working fine for a long time.
After further testing, it seems my problem is really with the {exp:weblog:category_heading} tag. I created a “test” template group and basically inserted the following code within the exp:weblog:entries tags of my template:
{if count == 1}
{exp:weblog:category_heading}<h1>{category_name}</h1>{/exp:weblog:category_heading}
{if category_request}This is a category request.{if:else}This is not a category request.
{/if}
{/if} {!-- if first entry --}
If you look at this page at http://bestwebconference.com/en/software-services/test/C34 (using the old notation) you can see that it correctly generates the <h1> header for the category.
If you use the new notation like this: http://bestwebconference.com/en/software-services/test/category/reviews/ EE detects that it is a category request, but there is no <h1> header.
The {exp:weblog:category_heading} tag does not appear to work the same if you use Category URL Titles. Is this intentional?