There is a problem with the EDID supplied by your DELL U2717D. The Nvidia driver interprets it as 2048x1080 being the maximum resolution for this monitor. On the other hand, the tool monitor-parse-edid spits out this for the EDID in your log file:
DELL U2717D
EISA ID: DEL40eb
EDID version: 1.3
EDID extension blocks: 1
Screen size: 59.7 cm x 33.6 cm (26.97 inches, aspect ratio 16/9 = 1.78)
Gamma: 2.2
Digital signal
Max video bandwidth: 250 MHz
HorizSync 30-88
VertRefresh 50-75
# Monitor preferred modeline (60.0 Hz vsync, 88.8 kHz hsync, ratio 16/9, 108 dpi)
ModeLine "2560x1440" 241.5 2560 2608 2640 2720 1440 1443 1448 1481 -hsync +vsync
# Monitor supported CEA modeline (60.0 Hz vsync, 67.5 kHz hsync, ratio 16/9, 81 dpi)
ModeLine "1920x1080" 148.5 1920 2008 2052 2200 1080 1084 1089 1125 +hsync +vsync
# Monitor supported CEA modeline (60.0 Hz vsync, 33.8 kHz hsync, interlaced, ratio 16/9, 81 dpi)
ModeLine "1920x1080" 74.25 1920 2008 2052 2200 1080 1084 1094 1125 +hsync +vsync Interlace
# Monitor supported CEA modeline (60.0 Hz vsync, 45.0 kHz hsync, ratio 16/9, 54 dpi)
ModeLine "1280x720" 74.25 1280 1390 1430 1650 720 725 730 750 +hsync +vsync
# Monitor supported CEA modeline (59.9 Hz vsync, 31.5 kHz hsync, ratio 3/2, 30x36 dpi) (bad ratio)
ModeLine "720x480" 27 720 736 798 858 480 489 495 525 -hsync -vsync
# Monitor supported CEA modeline (59.9 Hz vsync, 31.5 kHz hsync, ratio 3/2, 30x36 dpi) (bad ratio)
ModeLine "720x480" 27 720 736 798 858 480 489 495 525 -hsync -vsync
# Monitor supported CEA modeline (59.9 Hz vsync, 15.7 kHz hsync, interlaced, ratio 3.00, 61x36 dpi) (bad ratio)
ModeLine "1440x480" 27 1440 1478 1602 1716 480 488 494 525 -hsync -vsync Interlace
# Monitor supported CEA modeline (50.0 Hz vsync, 15.6 kHz hsync, interlaced, ratio 2.50, 61x43 dpi) (bad ratio)
ModeLine "1440x576" 27 1440 1464 1590 1728 576 580 586 625 -hsync -vsync Interlace
# Monitor supported CEA modeline (59.9 Hz vsync, 31.5 kHz hsync, ratio 4/3, 27x36 dpi) (bad ratio)
ModeLine "640x480" 25.175 640 656 752 800 480 490 492 525 -hsync -vsync
# Monitor supported CEA modeline (59.9 Hz vsync, 15.7 kHz hsync, interlaced, ratio 3.00, 61x36 dpi) (bad ratio)
ModeLine "1440x480" 27 1440 1478 1602 1716 480 488 494 525 -hsync -vsync Interlace
# Monitor supported CEA modeline (50.0 Hz vsync, 31.2 kHz hsync, ratio 5/4, 30x43 dpi) (bad ratio)
ModeLine "720x576" 27 720 732 796 864 576 581 586 625 -hsync -vsync
# Monitor supported CEA modeline (50.0 Hz vsync, 31.2 kHz hsync, ratio 5/4, 30x43 dpi) (bad ratio)
ModeLine "720x576" 27 720 732 796 864 576 581 586 625 -hsync -vsync
# Monitor supported CEA modeline (50.0 Hz vsync, 15.6 kHz hsync, interlaced, ratio 2.50, 61x43 dpi) (bad ratio)
ModeLine "1440x576" 27 1440 1464 1590 1728 576 580 586 625 -hsync -vsync Interlace
# Monitor supported CEA modeline (50.0 Hz vsync, 37.5 kHz hsync, ratio 16/9, 54 dpi)
ModeLine "1280x720" 74.25 1280 1720 1760 1980 720 725 730 750 +hsync +vsync
# Monitor supported CEA modeline (50.0 Hz vsync, 28.1 kHz hsync, interlaced, ratio 16/9, 81 dpi)
ModeLine "1920x1080" 74.25 1920 2448 2492 2640 1080 1084 1094 1125 +hsync +vsync Interlace
# Monitor supported CEA modeline (50.0 Hz vsync, 56.2 kHz hsync, ratio 16/9, 81 dpi)
ModeLine "1920x1080" 148.5 1920 2448 2492 2640 1080 1084 1089 1125 +hsync +vsync
# Monitor supported modeline (60.0 Hz vsync, 67.5 kHz hsync, ratio 16/9, 81 dpi)
ModeLine "1920x1080" 148.5 1920 2008 2052 2200 1080 1084 1089 1125 +hsync +vsync
# Monitor supported modeline (60.0 Hz vsync, 66.7 kHz hsync, ratio 1.90, 87x81 dpi) (bad ratio)
ModeLine "2048x1080" 147.18 2048 2096 2128 2208 1080 1083 1093 1111 -hsync +vsync
# Monitor supported modeline (60.0 Hz vsync, 45.0 kHz hsync, ratio 16/9, 54 dpi)
ModeLine "1280x720" 74.25 1280 1390 1430 1650 720 725 730 750 +hsync +vsync
# Monitor supported modeline (24.0 Hz vsync, 26.4 kHz hsync, ratio 1.90, 87x81 dpi) (bad ratio)
ModeLine "2048x1080" 58.23 2048 2096 2128 2208 1080 1083 1093 1099 -hsync +vsync
One way to rectify things is to create a custom EDID based on the line that says “Monitor preferred modeline” at the top of this output and then feed that EDID to the Nvidia driver with the CustomEDID option in xorg.conf.
Or you do it the old way with Modelines in xorg.conf.