Is it sourness from the first sip or is it an after taste?
I have a friend who noticed a sour after taste in my IPA. After trying two bottles of the IPA (bottled for over two months), he said that pouring less beer into his glass (leaving more yeast behind in the second bottle) made the sour aftertaste less intense. The temperature of the fermentation was controlled in the mid 60'S F and Safeale-05 has a pretty clean profile so I wouldn't blame temperature. I would say try tasting two bottles side by side , leaving more yeast behind in one and see it it helps. Maybe it is yeast bite.
For a Belgian I brewed I have also received comments about a sour after taste and was told that smacking the pack and adding it to the starter without giving it at least a day to swell up could lead to off flavors. How long before adding the yeast to your starter did you smack the pack (assuming the liquid yeast you ordered was the smack pack kind)?.
I hear that Belgians get better with age as well. Maybe leaving it in the bottle longer will help clean out the flavor.