Tiagoheineck,  To clarify, you are seeing the server say that ECC is now disabled? Seeing that is likely due to the Memory Mode being set to ECC, but with a dimm present in A1 it will disable that memory mode.  To populate the server in Adv ECC mode would need the dimms to be in slot A2, A3, and B2, and B3. If you set the servers Memory Mode to Optimizer then the dimms indeed would go to A1, A2, and B2, B3. To verify the mode the server is set to just access the BIOS (F2 on startup) and then under Memory Settings, verify what Memory Operating Mode is listed as. Lastly, although the R710 does support 16gb dimms, there are some processors that may not be compatible with using the 16gb dimms. Let me know what processor is installed and I can let you know compatibility.  Let me know.
We have upgrading memory of the r710, which slot configuration is the best? 4x16gb kingstonn ktd-pe313lv/16gb A1 - A2 , B1 - B2 ? this mode the server say which "ECC mode is disabled" 
R710 upgrade memory