Issue where soft-credits will not save
Great to hear you're using my extension!

Since the extension works but the form doesn't, that strongly suggests the problem is with this form and not the database.  From your video, it looks like a JavaScript issue.  Here's what I would look at:

Press F12 in your web browser to bring up the dev tools. 


Look at the "Console" tab. Are there errors that appear there on save when the save fails vs. succeeds?  If so, check out this post: Troubleshooting JavaScript in CiviCRM.  Also feel free to update your question with said errors.
Also check the "Network tab".  Find the HTTP POST request that appears when you press "Save".  Particularly, look at the "Response" subtab (I use Firefox, terminology may be a bit different with Chrome/Edge but it's all there).  There is likely a helpful clue there.