Does it happen when the command to heat is sent to the printer? Perhaps a circuit is blown on the printer. Check the G-Code Terminal for the commands being sent when the problem occurs: http://wiki.mattercontrol.com/OPTIONS/G-Code_Terminal
This is a known issue and it will be fixed in 1.5.1.
1.5.1 is currently in alpha (for windows only), so you can try it out now if you want to.
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