ScriptResource.axd error when deploying to a client: Sys.WebForms.PageRequestManagerServerErrorException

I'm at a loss here. We have an ASP.NET (4.6) Web Forms page that works perfectly on all of our servers plus several client servers. But right now I am deploying this app to a client, and when clicking on a button, nothing happens. In the console, I see this error:

Uncaught Error: Sys.WebForms.PageRequestManagerServerErrorException: Object reference not set to an instance of an object. at Function.Error$create [as create] (ScriptResource.axd?d=sscALjz_hfd6EF_1o1vKQqbw_llr4KBfiTDMmPsz-TYuJ07wQjrp5g4FNfrg311fIzrytx0r4q1RtfRlzyfes4648-ibd87UnYSIGA9p0mXUeuNSOaVYCQwVk__kM36nYdYc8eoKVnyH9_Y8EEla4Q2&t=ffffffffa66fe644:846) at PageRequestManager$_createPageRequestManagerServerError [as _createPageRequestManagerServerError] (ScriptResource.axd?d=wCQ5O5U4b4-FXZ8-P7CEgwaonx9gUcDjA09J5Azw0iL9Fk7ior92wbV5J_e_B62aHkamz3LXfRZSfi1yMw6lkilFxkc0_0uWvtsCca360k6N5cNy9YUEnsoF7o_L38nUuGDMTc_61gDnfypHWUm1Bw2&t=ffffffffa66fe644:764) at PageRequestManager$_parseDelta [as _parseDelta] (ScriptResource.axd?d=wCQ5O5U4b4-FXZ8-P7CEgwaonx9gUcDjA09J5Azw0iL9Fk7ior92wbV5J_e_B62aHkamz3LXfRZSfi1yMw6lkilFxkc0_0uWvtsCca360k6N5cNy9YUEnsoF7o_L38nUuGDMTc_61gDnfypHWUm1Bw2&t=ffffffffa66fe644:1653) at PageRequestManager$_onFormSubmitCompleted [as _onFormSubmitCompleted] (ScriptResource.axd?d=wCQ5O5U4b4-FXZ8-P7CEgwaonx9gUcDjA09J5Azw0iL9Fk7ior92wbV5J_e_B62aHkamz3LXfRZSfi1yMw6lkilFxkc0_0uWvtsCca360k6N5cNy9YUEnsoF7o_L38nUuGDMTc_61gDnfypHWUm1Bw2&t=ffffffffa66fe644:1414) at Array. (ScriptResource.axd?d=sscALjz_hfd6EF_1o1vKQqbw_llr4KBfiTDMmPsz-TYuJ07wQjrp5g4FNfrg311fIzrytx0r4q1RtfRlzyfes4648-ibd87UnYSIGA9p0mXUeuNSOaVYCQwVk__kM36nYdYc8eoKVnyH9_Y8EEla4Q2&t=ffffffffa66fe644:627) at ScriptResource.axd?d=sscALjz_hfd6EF_1o1vKQqbw_llr4KBfiTDMmPsz-TYuJ07wQjrp5g4FNfrg311fIzrytx0r4q1RtfRlzyfes4648-ibd87UnYSIGA9p0mXUeuNSOaVYCQwVk__kM36nYdYc8eoKVnyH9_Y8EEla4Q2&t=ffffffffa66fe644:4337 at raise (ScriptResource.axd?d=sscALjz_hfd6EF_1o1vKQqbw_llr4KBfiTDMmPsz-TYuJ07wQjrp5g4FNfrg311fIzrytx0r4q1RtfRlzyfes4648-ibd87UnYSIGA9p0mXUeuNSOaVYCQwVk__kM36nYdYc8eoKVnyH9_Y8EEla4Q2&t=ffffffffa66fe644:7247) at WebRequest$completed [as completed] (ScriptResource.axd?d=sscALjz_hfd6EF_1o1vKQqbw_llr4KBfiTDMmPsz-TYuJ07wQjrp5g4FNfrg311fIzrytx0r4q1RtfRlzyfes4648-ibd87UnYSIGA9p0mXUeuNSOaVYCQwVk__kM36nYdYc8eoKVnyH9_Y8EEla4Q2&t=ffffffffa66fe644:7251) at XMLHttpRequest.XMLHttpExecutor._onReadyStateChange (ScriptResource.axd?d=sscALjz_hfd6EF_1o1vKQqbw_llr4KBfiTDMmPsz-TYuJ07wQjrp5g4FNfrg311fIzrytx0r4q1RtfRlzyfes4648-ibd87UnYSIGA9p0mXUeuNSOaVYCQwVk__kM36nYdYc8eoKVnyH9_Y8EEla4Q2&t=ffffffffa66fe644:6856)

In going into the source of the error, which is in the file ScriptResource.axd, the error is being thrown here:

 _endPostBack: function PageRequestManager$_endPostBack(error, executor, data) {
        if (this._request === executor.get_webRequest()) {
            this._processingRequest = false;
            this._additionalInput = null;
            this._request = null;
        }

        var eventArgs = new Sys.WebForms.EndRequestEventArgs(error, data ? data.dataItems : {}, executor);
        Sys.Observer.raiseEvent(this, "endRequest", eventArgs);
        if (error && !eventArgs.get_errorHandled()) {
            throw error;
        }
    },

The page uses an Update Panel. From this thread:

PageRequestManagerServerErrorException object reference not set... ASP.net AJAX

It looks like the problem is that the _OnRowCreated event was firing before the Page_load. The OP answered his own question, but I'm not making any sense of it.

And, WHY is it happening only on this one customer's server? Why have we not seen this error on any of our servers? The version of .NET is the same, we made sure of that.

EDIT:

I'm not even using _OnRowCreated anywhere in the page in question