Try with 5.0.1 (not 5.0 because it will crash when you point to vSphere 5.5!). 5.1 might also work.
Longer answer: There is no explicit disabling of older GOS version in the code and they usually can be converted albeit not supported. However Ubuntu 8 is known not to work (don't remember which version since). Also Converter is usually able to point to a newer vSphere version as a destination (after fixing the bug in 5.0). Keep in mind neither of these 'usuals' is supported but with some luck you may succeed. If not, the workaround is to convert to some older ESX and then to the real destination.
HTH
Plamen