Cannot activate previous version of parallels

I have parallels from version 3 on. Then I upgraded to 4 and then to 5.

A few days ago I tried version 6 (trial) but had problems with sound in Ubuntu, so I decided to go back to version 5.

I uninstalled 6 and installed 5. It asked for my activation number, but my number (which I bought) does not work anymore.

Please help.
 
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