Someone asked me this when I said I was transitting at Doha and they said its needed in Dubai. I dont know if thats true but they were under that impressiion
I know this was half a year ago, but i am currently having similar issues. Submitted the application and it says payment pending, but I press "pay now" and get a server error.
Do they really only require proof of 20,000 baht? Last year I popped to Mae Sai for a border bounce and was concerned I might not have enough money if they wanted proof of funds but 20,000 is easy!