I can understand how frustrating this must be. There could be a whole lot of reasons why this is happening, but our developers usually need to see the error your are seeing to be able to determine the cause of the error.
However, on your side, I would check in ClientZone whether the TopUp has registered before trying it again, if the error was after the order was completed (but maybe timed out during the payment stage) you may have already received and been billed for the topup.
Before trying again, make sure that your clear your browser cache and also clear cookies. Maybe even restart your web browser. Give it another try, and if you get another error, please take a screenshot and then email us on email@example.com with the steps you followed and the error you got (as well as your account details) and we'll have our Dev team look into this for you.