Your comments

After some research, i located the problem with the js/login.js file near line 85

var responseText = frameDoc.body.innerHTML;
            if (responseText.length == 0) {
                return false;
            try {
                var rs = Ext.util.JSON.decode(responseText);
            } catch (er) {
                if (confirm(FR.T('Unexpected server reply. Press "OK" to display it.'))) {
                return false;

On first line var responseText = frameDoc.body.innerHTML;, responseText seems empty and therefore cannot be decoded.

Not sure but it might come from an iframe issue ? Is Chrome restricting using script inside iframe ?

Some help would be apreciated