The reason it looks like the log has disappeared is when the error happens it puts the entire scripts code in the log. The script is so big that it won't fit into the amount of memory setup for the log file so it clears the memory. If you look in the actual log file in the config directory the entries are still there.
Now to the error.
It is saying that XMLDocument is not defined.
Look in the weatherxml docs on page 1. There is a section for ScriptingReferences. That line needs to be added to the HomeSeer settings.ini file. It has to be in the [Settings] section.
Also check to see if you have ScriptingReferences already in the file. If so, then just add the values from the weatherxml documentation.
HS 188.8.131.522, HSPhone
My HS3 Plugins: CFHSExtras, Random, Restart, Tracker, WeatherXML, PanaBluRay
Other Plugins In Use: APCUPSD, EasyTrigger, FBitSeer, HSTouch Server, Kinect, PHLocation, Pushover, RFXCom, UltraGCIR3, UltraMon3, UltraPioneerAVR3, X10, Z-Wave
Hardware: Schlage Z-Wave Lever Lock x2, Way2Call Hi-Phone, RFXCom RFXrec433 Receiver, WGL 800, TI-103, Z-Net, Pioneer 1120, Pioneer 1021, Pioneer 1122, Panasonic BDT-110, Panasonic BDT-210 x2