I have been using Coinb.in and I just tried doing a withdraw with my segwit adress and i can't, i get the following error:
No, its me who used the source code to make my own "coinbin"
I've checked the source of your own coinb.in and confirmed that all the necessary javascript are there, but you seem to have removed the whole "
Donation" element.
That small edit had impacted the code for the transaction generation.
I have also successfully reproduced the issue by removing the same element.
And solved the issue by just hiding the element instead of removing it:
Just add this in the element of that donation amount:
style="visibility:hidden"
Then edit the default value to "0".
If you wan't to totally disable it, tinker with the the javascript(s), it's your site.