I have similar problem with PHP-Push-2 project.
When I try contact sync, the server response is 403 on REPORT query.

I've changed query in PROPFIND and sync seems work correctly.... but I think this is not a solution.

@MichelR
Add patches to this bug report

Any updates on the patches?

Please find the file "icewarp_403.patch"
I don't know if the added line "this.abort()" is correct, or if "abort()" needs to be modified to handle this new case.

In a similar way, I have a question about the "onServerHashQueryComplete" method (/chrome/content/sogo-connector/general/sync.addressbook.groupdav.js) :

current version :

onServerHashQueryComplete: function(status, jsonResponse) {

    ...
    if (jsonResponse) {
        if (status > 199 && status < 400) {
            ...
        }
        else
            this.abort();
    }
    else
        dump("onServerHashQueryComlete: the server returned an empty response\n");
},

Shouldn't it need a "abort()" when there is no json response ?

onServerHashQueryComplete: function(status, jsonResponse) {

    ...
    if (jsonResponse) {
        if (status > 199 && status < 400) {
            ...
        }
        else
            this.abort();
    }
    else
    {
        dump("onServerHashQueryComlete: the server returned an empty response\n");
        this.abort();
    }
},

Patch pushed: https://github.com/inverse-inc/sogo-connector.tb24/commit/e90acfbe7de32992e61108a446fd34eed9978765

I don't understand your command #5616 - that's exactly what the patch does!