Have you tried ftping to cache and reading the myDEBUG ......log report?
Might give you a good starting point.

I also remember resetting all the caches after my 'practising' helped ;)

I've found a similar problem when clicking from a manufacturers side box if that manufacturer only has one product, depending on your main settings, in particular what you set to display a manufacturer must have at least two products or ssu falls over.

If I had a query it would be why am I still seeing for example url.com/category-name.C.2-4 in my site links when I surf but using url.com/category-name as a clickable link works and takes the visitor to url.com/category-name.C.2-4

I can see benefit to do it this way but ... is that how SSU works?

Some clearer documentation would really help but so far ... it looks like it's working.