[ubuntu] Can't stay logged in to flash based site

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http://ubuntuforums.org – I have recently had problems with www.mycmc.co.uk. After a maximum of ten minutes (often three or four) I am logged out when using Ubuntu or Ubuntu 2D. When using Windows from the same laptop I can stay logged in all day. The problem is the same with Chrome and Firefox on two different computers. I have cleared cookies, tried three versions of flash (via Flashaid)and removed all extensions/addins, but the only thing that works is Windows. Any ideas? (Hardware)