Some users seem to have no webcam support with current settings. Send on console
/usr/bin/v4l2-ctl --list-formats-ext -d /dev/video0
to see what modes your webcam supports. Then edit /usr/local/Repetier-Setup/etc/webcam.conf to use one of the resolution/frame rate combination that your webcam supports. After that reboot the Pi so the changes take effect.
An other problem may occur if you have multiple webcams. If your webcam only supports YUYV mode and not MJPG it will send all data uncompressed so it puts a high load on your usb port. As a result you can only handle one webcam per USB port.
Last issue with multiple identical webcams is that they can have the same name so you only see one in our selection list. In that case follow these instructions.