I have encountered the following issue while testing a website that uses HTTP/2:
Normal requests works but the the requests that goes through AutoRepeater get a HTTP/1.1 505 HTTP Version Not Supported
Example:
Original:

Autorepeater modified:

And sending this autorepeater modified request to the Repeater let's me send a request that is working:
Autorepeater modified request but send through the burp Repeater:
