It could be that the PayPal site (or whichever site you were accessing PayPal through) simply doesn't like Silk. (I've visited a website this evening which told me that it couldn't function properly because I was using an out-of-date version of Internet Explorer - despite the fact that I was actually using the latrst version of Firefox!).
Try . . .
1. Clearing the cache and cookies from Silk:
http://tinyurl.com/ydxf5jml
2. Checking that the software for your Fire HD is up to date:
Instructions for 2nd Generation models:
http://tinyurl.com/ycx9zxk4
3rd Generation:
http://tinyurl.com/yc9d5jne
4th Generation:
http://tinyurl.com/y7ohu2gv
3. Using a different browser:
https://www.technipages.com/kindle-fire-silk-browser-alternatives