Please remove the constraint similar to the following from your output UCF.
QDRII+
#half cycle constraint for ISERDES outputs when clock inversion is used
NET "u_user_top/*/iserdes_*d*_int" TNM = TNM_iserdes_outputs;
TIMESPEC "TS_clk_rd_half_cycle" = FROM "TNM_iserdes_outputs" TO "FFS" TS_clk_rd/2;
RLDRAMII
#half cycle constraint for ISERDES outputs when clock inversion is used
NET "*iserdes_*d*_int" TNM = iserdes_outputs;
TIMESPEC "TS_clk_rd_half_cycle" = FROM "iserdes_outputs" TO "FFS" TS_clk_rd/2;