Repeating admin-ajax.php not found error in admin

I seem to be getting a repeating 404 error in the Chrome console when in wordpress admin very occasionally it gets marked as a 500 error for the same line, perhaps is something to do with an authorisation check as surrounding calls to that file are. They build up over time even when no action is being taken. The error is:

load-scripts.php?c=1&load[]=jquery-core,jquery-migrate,utils,jquery-ui-core,jquery-ui-widget,jquery-ui-mouse,moxiejs,plupload&ver=5.1:formatted:4208

POST .php 404 (Not Found)

On clicking the link the problem line 4208 (red cross) in the code shows as:

  g.send(b.hasContent && b.data || null),

I read around on this and someone suggested for a similar error line that the script is not being loaded under https while the site is https. However having scanned through every reference I can find is all using https and I even tried forcing https with htaccess since.

To be honest I'm not sure what this relates to and is tricky to see where the sections of code begin and end, some more of the surrounding code of the above line is:

l.cors = !!gc && "withCredentials"in gc,
gc = l.ajax = !!gc,
gc && n.ajaxTransport(function(b) {
    if (!b.crossDomain || l.cors) {
        var c;
        return {
            send: function(d, e) {
                var f, g = b.xhr(), h = ++ec;
                if (g.open(b.type, b.url, b.async, b.username, b.password),
                b.xhrFields)
                    for (f in b.xhrFields)
                        g[f] = b.xhrFields[f];
                b.mimeType && g.overrideMimeType && g.overrideMimeType(b.mimeType),
                b.crossDomain || d["X-Requested-With"] || (d["X-Requested-With"] = "XMLHttpRequest");
                for (f in d)
                    void 0 !== d[f] && g.setRequestHeader(f, d[f] + "");
                g.send(b.hasContent && b.data || null),
                c = function(a, d) {
                    var f, i, j;
                    if (c && (d || 4 === g.readyState))
                        if (delete fc[h],
                        c = void 0,
                        g.onreadystatechange = n.noop,
                        d)
                            4 !== g.readyState && g.abort();
                        else {
                            j = {},
                            f = g.status,
                            "string" == typeof g.responseText && (j.text = g.responseText);
                            try {
                                i = g.statusText
                            } catch (k) {
                                i = ""
                            }
                            f || !b.isLocal || b.crossDomain ? 1223 === f && (f = 204) : f = j.text ? 200 : 404
                        }
                    j && e(f, i, j, g.getAllResponseHeaders())
                }
                ,
                b.async ? 4 === g.readyState ? a.setTimeout(c) : g.onreadystatechange = fc[h] = c : c()
            },
            abort: function() {
                c && c(void 0, !0)
            }
        }
    }
});

If anyone knows what this might relate to and if it's a serious problem I'd be grateful for some feedback. Thanks

Edit: am finding this repeating (for various .php files and referer url endings) in error.log

[Sat Mar 09 00:38:06 2019] [warn] [client w.x.y.z] (104)Connection reset by peer: mod_fcgid: error reading data from FastCGI server, referer: /

[Sat Mar 09 00:38:06 2019] [error] [client w.x.y.z] Premature end of script headers: admin-ajax.php, referer: /

So perhaps is related, this issue is looked at here: