Unfortunately, as you do use a Ubuntu version which has been officially dropped since April 2012 and therefore also a quite old Diodon version, I won't really be able to reproduce this issue.
If you want to stick to this old version though what you could try to do is delete the Diodon history (rm ~/.local/share/diodon/storage.xml). That might solve the issue.
I would highly recommend you though to update to a still supported Ubuntu version and also a new Diodon version. Let me know whether the issue appears on a newer version as well.
Thanks for updating the issue.
Unfortunately, as you do use a Ubuntu version which has been officially dropped since April 2012 and therefore also a quite old Diodon version, I won't really be able to reproduce this issue.
If you want to stick to this old version though what you could try to do is delete the Diodon history (rm ~/.local/ share/diodon/ storage. xml). That might solve the issue.
I would highly recommend you though to update to a still supported Ubuntu version and also a new Diodon version. Let me know whether the issue appears on a newer version as well.
Thanks.