@Chris D the 2.4.0 version of this addon ships with 
psr/http-message v2.0, while XenForo 2.2.x ships with 
psr/http-message v1.0.1
Addon v2.4.0:
	
	
	
		JSON:
	
	
		            "name": "psr/http-message",
            "version": "2.0",
	 
 
XenForo v2.2.x:
	
	
	
		JSON:
	
	
		             "name": "psr/http-message",
            "version": "1.0.1",
	 
 
The difference between the two being the function definition in the interface:
	
	
	
		PHP:
	
	
		@@ -37,7 +38,7 @@ interface UriInterface
     * @see https://tools.ietf.org/html/rfc3986#section-3.1
     * @return string The URI scheme.
     */
    public function getScheme();
    public function getScheme(): string;
	 
 
I believe that this is what is causing the conflict.
I note that v2.3.0 of this addon did not ship with 
psr/http-message at all, so would not have caused any issues.
Can I suggest that you re-release this addon with 
psr/http-message locked to v1.0.1 for compatibility with XF 2.2.x ?