Rule Violation (RTSTAT-6) Partial Conflict issue on Vivado rev 2015.4

For Vivado 2015.4, I hit an error like the following when click "Generate Bitstream" after implementation is done.
ERROR: [DRC 23-20] Rule violation (RTSTAT-6) Partial Conflict - 18640 net(s) have a partial conflict. The problem bus(es) and/or net(s) are .....

The errors occured when I use the implemenation strategy of "Vivado Implementation Defaults". Also tried "Performance_Explore", it does not help. It is found that when I try the implementation strategy of "Area_Explore", the RTSTAT-6 disappeared and bitstream can be generated successfully after implementation.