I'm seeing the same problem as robin.puga. I looked at the code (which is a little messy) and I found that the problem, in my case at least, is with uc_worldquote_match_weight().

I don't use weight at all on one particular store, which means all weights are 0. So I defined all my rules as flatrate. The if statements in uc_worldquote_match_weight() doesn't seem to handle this case very well.

I simply commented out the call to that function in uc_worldquote_quote() and now it works. Obviously not a good solution for people in general, but somewhere there lies the problem.