What to do with a dysfunctional Romisen RC-T601 II XM-L U2

That is the same circuitry, but the LED is not as bright, but does show colours better, if you want more brightness i would get this chip instead
http://www.mtnelectronics.com/index.php?route=product/product&product_id=387&search=xp-l

The store is run by RMM (Richard), he is a member at BLF and has his own thread where you can ask questions