Yeah, this one's common. Your RO system is working too well. It strips out almost everything, including the minerals that give water its taste.
DIAGNOSIS: The water's got extremely low Total Dissolved Solids (TDS).
FIX: It's a feature, not a bug. The system is designed to produce very pure water. Water with a TDS level below 300 mg/liter is rated as having excellent palatability, but some people find it tastes flat or insipid.
IF THAT DIDN'T WORK: If the taste is truly off (sour, chemical, etc.), that's a different issue. Check your filters. A spent post-carbon filter or a contaminated membrane can cause bad tastes.
HEADS UP: If you really miss the mineral taste, you can add a remineralization filter as an after-market upgrade to your system. Otherwise, you get used to the clean taste.