Can't place differential Pins, seems like a glitch

Hi,

I want to use a memory controller and for that I was assigning all necessary pins. Since I've built the project several times already (I had to start a new project FYI) I know how the Pins have to be assigned and it was always working and is working with these settings in another project.

However I get the following Errors:

Error (169290): Can't place differential I/O positive pin DDR3_CK_n at a differential I/O negative location AA15(PAD_120)

Error (169290): Can't place differential I/O positive pin DDR3_DQS_n[0] at a differential I/O negative location W16(PAD_136)

Error (169290): Can't place differential I/O positive pin DDR3_DQS_n[1] at a differential I/O negative location W17(PAD_152)

Error (169290): Can't place differential I/O positive pin DDR3_DQS_n[2] at a differential I/O negative location AA18(PAD_168)

Error (169290): Can't place differential I/O positive pin DDR3_DQS_n[3] at a differential I/O negative location AD19(PAD_184)

When I switch for example DDR3_CK_n with the respective positive Pin DDR3_CK_p, it gives the exact same error message for DDR3_CK_p.

Does anyone have a clue what I could do to fix this?

Thanks in advance,:)

Z