I am sorry to hear that you are experiencing some troubles with the load test dynamic targets. We have recently made some changes and improvements to the overall behavior of the load test configuration. Please update to the latest Service pack version 2019.2.1030.x, if you have not done it already, and let me know if that makes any difference.
I tried to reproduce the issue on my end with a load test against a public application, which is data driven with external Excel file. The POST request holds 2 and more custom dynamic targets and I was able to data bind them and save those changes. I need your help to try and find what is causing the issue. Please share a sample Test Studio project, which simulates the issue and make sure to add the external source to the project and reproduce the issue. Then zip it and attach it to your next reply.
The "Unhandled Exception" seems to happen when you click on the "Save" button, but there is no clear reason for that in the log. I hope that the above mentioned sample project will help me reproduce the issue and investigate it further.
I am looking forward to hearing from you.