The synthesis does not stop when the error is issued and continues to run normally.
This issue does not affect the synthesis and will be fixed in the 5.1i software release. The error can safely be ignored.
The directory %XILINX%\vhdl\ieee\ should be cleaned from the temporary files xil_<xx> from time to time. When you clean the directory, however, do NOT delete the files "hdpdeps.ref", "vhdllib.ref" and "arch.vho". These files are needed for synthesis, which will not work any longer if these files are deleted.