CORS Error sometimes and sometimes not at POST Request

Hello, I have a Flask Python Backend at my domain. Sometimes I get a good response, sometimes I get a Cors Error with Error Message: Response to Preflight Request doesnt pass access control check

I created a Worker with following Code:

/*
CORS Anywhere as a Cloudflare Worker!
(c) 2019 by Zibri (www.zibri.org)
email: zibri AT zibri DOT org
https://github.com/Zibri/cloudflare-cors-anywhere
*/

/*
whitelist = [ "^http.?://www.zibri.org$", "zibri.org$", "test\\..*" ];  // regexp for whitelisted urls
*/

blacklist = [ ];           // regexp for blacklisted urls
whitelist = [ ".*" ];     // regexp for whitelisted origins

function isListed(uri,listing) {
    var ret=false;
    if (typeof uri == "string") {
        listing.forEach((m)=>{
	          if (uri.match(m)!=null) ret=true;
        });
    } else {            //   decide what to do when Origin is null
    	  ret=true;    // true accepts null origins false rejects them.
    }
    return ret;
}

addEventListener("fetch", async event=>{
    event.respondWith((async function() {
        isOPTIONS = (event.request.method == "OPTIONS");
        var origin_url = new URL(event.request.url);

        function fix(myHeaders) {
            //            myHeaders.set("Access-Control-Allow-Origin", "*");
            myHeaders.set("Access-Control-Allow-Origin", event.request.headers.get("Origin"));
            if (isOPTIONS) {
                myHeaders.set("Access-Control-Allow-Methods", event.request.headers.get("access-control-request-method"));
                acrh = event.request.headers.get("access-control-request-headers");
                //myHeaders.set("Access-Control-Allow-Credentials", "true");

                if (acrh) {
                    myHeaders.set("Access-Control-Allow-Headers", acrh);
                }

                myHeaders.delete("X-Content-Type-Options");
            }
            return myHeaders;
        }
        var fetch_url = decodeURIComponent(decodeURIComponent(origin_url.search.substr(1)));

        var orig = event.request.headers.get("Origin");
        
        var remIp = event.request.headers.get("CF-Connecting-IP");

        if ((!isListed(fetch_url, blacklist)) && (isListed(orig, whitelist))) {

            xheaders = event.request.headers.get("x-cors-headers");

            if (xheaders != null) {
                try {
                    xheaders = JSON.parse(xheaders);
                } catch (e) {}
            }

            if (origin_url.search.startsWith("?")) {
                recv_headers = {};
                for (var pair of event.request.headers.entries()) {
                    if ((pair[0].match("^origin") == null) && 
			(pair[0].match("eferer") == null) && 
			(pair[0].match("^cf-") == null) && 
			(pair[0].match("^x-forw") == null) && 
			(pair[0].match("^x-cors-headers") == null)
		    ) recv_headers[pair[0]] = pair[1];
                }
		    
                if (xheaders != null) {
                    Object.entries(xheaders).forEach((c)=>recv_headers[c[0]] = c[1]);
                }

                if (typeof cookies == "undefined") {
                    recv_headers["Cookie"] = cookies;
                }
                if (host != null) {
                    recv_headers["Host"] = host;
                }

                newreq = new Request(event.request,{
                    "redirect": "follow",
                    "headers": recv_headers
                });

                var response = await fetch(fetch_url,newreq);
                var myHeaders = new Headers(response.headers);
                cors_headers = [];
                allh = {};
                for (var pair of response.headers.entries()) {
                    cors_headers.push(pair[0]);
                    allh[pair[0]] = pair[1];
                }
                cors_headers.push("cors-received-headers");
                myHeaders = fix(myHeaders);

                myHeaders.set("Access-Control-Expose-Headers", cors_headers.join(","));

                myHeaders.set("cors-received-headers", JSON.stringify(allh));

                if (isOPTIONS) {
                    var body = null;
                } else {
                    var body = await response.arrayBuffer();
                }

                var init = {
                    headers: myHeaders,
                    status: (isOPTIONS ? 200 : response.status),
                    statusText: (isOPTIONS ? "OK" : response.statusText)
                };
                return new Response(body,init);

            } else {
                var myHeaders = new Headers();
                myHeaders = fix(myHeaders);

                if (typeof event.request.cf != "undefined") {
                    if (typeof event.request.cf.country != "undefined") {
                        country = event.request.cf.country;
                    } else
                        country = false;

                    if (typeof event.request.cf.colo != "undefined") {
                        colo = event.request.cf.colo;
                    } else
                        colo = false;
                } else {
                    country = false;
                    colo = false;
                }

                return new Response(
                	"CLOUDFLARE-CORS-ANYWHERE\n\n" + 
                	"Source:\nhttps://github.com/Zibri/cloudflare-cors-anywhere\n\n" + 
                	"Usage:\n" + origin_url.origin + "/?uri\n\n" +
			"Donate:\nhttps://paypal.me/Zibri/5\n\n" +
                	"Limits: 100,000 requests/day\n" + 
                	"          1,000 requests/10 minutes\n\n" + 
                	(orig != null ? "Origin: " + orig + "\n" : "") + 
                	"Ip: " + remIp + "\n" + 
                	(country ? "Country: " + country + "\n" : "") + 
                	(colo ? "Datacenter: " + colo + "\n" : "") + "\n" + 
                	((xheaders != null) ? "\nx-cors-headers: " + JSON.stringify(xheaders) : ""),
                	{status: 200, headers: myHeaders}
                );
            }
        } else {

            return new Response(
                "Create your own cors proxy</br>\n" + 
                "<a href='https://github.com/Zibri/cloudflare-cors-anywhere'>https://github.com/Zibri/cloudflare-cors-anywhere</a></br>\n" +
                "\nDonate</br>\n" +
                "<a href='https://paypal.me/Zibri/5'>https://paypal.me/Zibri/5</a>\n",
                {
                    status: 403,
                    statusText: 'Forbidden',
                    headers: {
                        "Content-Type": "text/html"
                    }
                });
        }
    }
    )());
});

Also I created a Worker Route with the route markb.de/* and my created worker script. Sometimes I get the response from server back and sometimes I get a Cors Error. How can I fix this?

My trace says following:

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