The login/new client field is not mapped to any client value, however, I now spotted that you do have 2 controls on the first page which are mapped to client values. I apologise for the 'red herring'.
The problem here is not so much that the product is not found, it is that the value you select in that dropdown list gets set back to the default value when you click on the 'next' button (and the default value has no product associated with it). If you select the option that contains the ø in the value, then click next, then click back, it has reverted back to the first option in the list.
The workaround is simply to only use latin characters in option values. This should not be a problem as the client never sees the option value, only the description, which can include non-latin characters. Still, I will update the code for the next release to allow for encoded characters to be recognised in option values.