We can't load this chat, because something went wrong - Salesforce Chat Issue

1. Check the OWD on Chat Transcript Object.
If it is set to private, then you cannot view the Chat Transcript unless you are the owner or it is shared with you(Manual Sharing, Role Hierarchy, Sharing Rules, etc.)

2. Inspect in your browser.
a. Right Click on your browser and select Inspect.
b. Click Console tabs.
c. Check whether any custom Lightning components added are throwing any exceptions. In this case, remove the custom components added and try to view.

3. Query Body from Chat Transcript (LiveChatTranscript) object to check whether it is blank or null.

SELECT Body FROM LiveChatTranscript WHERE Id = '<Id of the Chat Transcript Record>'