Problem with uploading a sketch

Hello. Information above:

OS: Ubuntu 11.04 x64
Arduino IDE: 1.0 (x64)
Arduino device: Arduino UNO R3
Problem:
I was using arduino uno for a project which has to do with PollSerial communication. I have also attached on it the Video Experimenter Shield (Video Experimenter: Arduino shield that lets you do all kinds of experiments with video).
When I am trying to upload a new sketch I always detach the VE from arduino.
2 weeks ago, I could not upload a sketch anymore to the arduino. The result message from the arduino was: Error opening serial port '/dev/ttyACM0'. Then while searching on google,
I decided to buy one more Arduino UNO R3 device. When I first tried to upload a sketch
the result is the same. "Error opening serial port '/dev/ttyACM0'."
The error from IDE is this:

processing.app.SerialException: Error opening serial port '/dev/ttyACM0'.
	at processing.app.Serial.<init>(Serial.java:146)
	at processing.app.Serial.<init>(Serial.java:76)
	at processing.app.debug.Uploader.flushSerialBuffer(Uploader.java:75)
	at processing.app.debug.AvrdudeUploader.uploadViaBootloader(AvrdudeUploader.java:88)
	at processing.app.debug.AvrdudeUploader.uploadUsingPreferences(AvrdudeUploader.java:66)
	at processing.app.Sketch.upload(Sketch.java:1664)
	at processing.app.Sketch.exportApplet(Sketch.java:1624)
	at processing.app.Sketch.exportApplet(Sketch.java:1578)
	at processing.app.Editor$DefaultExportHandler.run(Editor.java:2314)
	at java.lang.Thread.run(Thread.java:679)
Caused by: gnu.io.UnsupportedCommOperationException: Invalid Parameter
	at gnu.io.RXTXPort.setSerialPortParams(RXTXPort.java:171)
	at processing.app.Serial.<init>(Serial.java:136)
	... 9 more
processing.app.debug.RunnerException: Error opening serial port '/dev/ttyACM0'.
	at processing.app.debug.Uploader.flushSerialBuffer(Uploader.java:99)
	at processing.app.debug.AvrdudeUploader.uploadViaBootloader(AvrdudeUploader.java:88)
	at processing.app.debug.AvrdudeUploader.uploadUsingPreferences(AvrdudeUploader.java:66)
	at processing.app.Sketch.upload(Sketch.java:1664)
	at processing.app.Sketch.exportApplet(Sketch.java:1624)
	at processing.app.Sketch.exportApplet(Sketch.java:1578)
	at processing.app.Editor$DefaultExportHandler.run(Editor.java:2314)
	at java.lang.Thread.run(Thread.java:679)

Can you help me??
Thanks