[SalesForce] System.HttpResponse[Status=Not Found, StatusCode=404]

This webservice has been running perfectly for almost a year now.

However today I am seeing this error

|System.HttpResponse[Status=Not Found, StatusCode=404]

11:08:55.0 (509386280)|FATAL_ERROR|System.JSONException: Unexpected character ('d' (code 100)): expected a valid value (number, String, array, object, 'true', 'false' or 'null') at input location [1,2]

Here's my code

        HttpRequest request = new HttpRequest();
        HttpResponse res = new HttpResponse();
        Http http = new Http(); 
        request.setMethod(httpVerb);
        request.setEndpoint(host + endpoint);   
        request.setHeader ('Date', timestamp);
        request.setHeader('Authorization', rawSignature);
        request.setTimeout(120000);
        system.debug(System.now()+'-- Send Request To Server --');
        res = http.send(request);
        **system.debug(System.now()+'-- get Response --'+res.getBody());**
        return res.getBody();




    public static List<Map<string, integer>> getChartMap(String accountid){

        Account accObj = [SELECT Id,External_ID__c FROM Account where Id =: accountid];
        system.debug(System.now()+'-- getAPIData called with externalID From GetChartMap Method --');
        String jsonData = getAPIData(accObj.External_ID__c );
        system.debug(System.now()+'-- get Response in getChartMap method --'+jsonData);
        List<Map<string, integer>> finalMap = new List<Map<string, integer>>();        
        Integer currenMonth = Date.Today().Month();
        **Map<String, Object> results = (Map<String, Object>)JSON.deserializeUntyped(jsonData);**

        ....

    }

Debug Logs

11:08:55.0 (507563738)|CALLOUT_RESPONSE|[66]|**System.HttpResponse[Status=Not Found, StatusCode=404]**
11:08:55.0 (507598672)|HEAP_ALLOCATE|[66]|Bytes:34
11:08:55.0 (507721573)|VARIABLE_ASSIGNMENT|[66]**|res|"System.HttpResponse[Status=Not Found,** StatusCode=404]"|0x494d4d21
11:08:55.0 (507732287)|STATEMENT_EXECUTE|[67]
11:08:55.0 (507784731)|HEAP_ALLOCATE|[67]|Bytes:56
11:08:55.0 (507833394)|HEAP_ALLOCATE|[67]|Bytes:19
11:08:55.0 (507972732)|HEAP_ALLOCATE|[67]|Bytes:18
11:08:55.0 (508026872)|HEAP_ALLOCATE|[67]|Bytes:21
11:08:55.0 (508073145)|HEAP_ALLOCATE|[67]|Bytes:58
11:08:55.0 (508093952)|USER_DEBUG|[67]|**DEBUG|2020-04-28 15:08:56-- get Response --default backend - 404**
11:08:55.0 (508106389)|STATEMENT_EXECUTE|[68]
11:08:55.0 (508136746)|HEAP_ALLOCATE|[68]|Bytes:21
11:08:55.0 (508153645)|METHOD_EXIT|[76]|01p2S000003FgbV|GraphController.getAPIData(String)
11:08:55.0 (508164831)|VARIABLE_SCOPE_BEGIN|[76]|jsonData|String|false|false
11:08:55.0 (508187910)|VARIABLE_ASSIGNMENT|[76]|jsonData|"default backend - 40 (1 more) ..."

USER_DEBUG|[77]|DEBUG|2020-04-28 15:08:56-- get Response in getChartMap method --default backend - 404
11:08:55.0 (508280487)|STATEMENT_EXECUTE|[78]
11:08:55.0 (508319018)|HEAP_ALLOCATE|[78]|Bytes:4
11:08:55.0 (508522669)|VARIABLE_SCOPE_BEGIN|[78]|finalMap|List<Map<String,Integer>>|true|false
11:08:55.0 (508587328)|HEAP_ALLOCATE|[EXTERNAL]|Bytes:4
11:08:55.0 (508607603)|VARIABLE_ASSIGNMENT|[78]|finalMap|[]|0x3371fc3f
11:08:55.0 (508615451)|STATEMENT_EXECUTE|[79]
11:08:55.0 (508670037)|HEAP_ALLOCATE|[79]|Bytes:4
11:08:55.0 (508699049)|VARIABLE_SCOPE_BEGIN|[79]|currenMonth|Integer|false|false
11:08:55.0 (508708582)|HEAP_ALLOCATE|[79]|Bytes:4
11:08:55.0 (508719881)|VARIABLE_ASSIGNMENT|[79]|currenMonth|4
11:08:55.0 (508726337)|STATEMENT_EXECUTE|[80]
11:08:55.0 (508789017)|HEAP_ALLOCATE|[80]|Bytes:24
11:08:55.0 (508806983)|SYSTEM_METHOD_ENTRY|[1]|JSON.JSON()
11:08:55.0 (508811815)|STATEMENT_EXECUTE|[1]
11:08:55.0 (508821367)|SYSTEM_METHOD_EXIT|[1]|JSON
11:08:55.0 (508843169)|METHOD_ENTRY|[80]||System.JSON.deserializeUntyped(String)
11:08:55.0 (509080864)|METHOD_EXIT|[80]||System.JSON.deserializeUntyped(String)
11:08:55.0 (509222994)|**FATAL_ERROR|System.JSONException: Unexpected character ('d' (code 100)): expected a valid value (number, String, array, object, 'true', 'false' or 'null') at input location [1,2]**