I know coinb.in uses javascript library's that run completely from your local machine, these library's use api's to query for unspent outputs funding a certain address.
Maybe there's a timeout from these api calls from your local machine? Could be the api provider, your isp or your computer/laptop/cellphone.
You can try to look at the console when you try to create a transaction?
You could also visit
https://coinb.in/#settings => Unspent outputs and select a different provider and see if that fixes your issue...