Arduino IDE not connecting to microcontroller

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http://askubuntu.com – I get this error when trying to connect to an Arduino through a USB serial connection. I'm using the Arduino IDE 1.0.1 and the 64bit version of Ubuntu 12.04. This has been a reoccurring problem since 10.04 and happens to a few other programs that use a serial connection too. I have no problem getting serial data from the Arduino using Python or Screen. The Arduino IDE seems to work just fine otherwise. processing.app.SerialException: Error opening serial port '/dev/ttyACM0'. at processing.app.Serial.<init>(Serial.java:178) at processing.app.Serial.<init>(Serial.java:92) at p (HowTos)