After a recent upgrade of the driver (and unfortunately I do not recall the old version) X always fail to start with the error both with the current and latest long term supported drivers:
[ 17.967] (EE) NVIDIA(0): DisplayPort validation failed for MetaMode "DFP-5:2560x1600"
Restarting slim (xdm manager) usually does not work either. however, if I power cycle the monitor it seems that I at times getting X going. I attached both a failed X session and on that works.