We’ve got several reports from users that they have problems with popup login form, all those issues happen on hosting that are using nginx server (this doesn’t happen or apache and other servers), we are not sure about the source of that problem, but in few cases the hosting providers have been able to fix it by either increasing  proxy-buffer size or by disabling/modifying ModSecurity to allow the ajax login call. If you are able to get help with your hosting provider, please contact them and describe that the ajax call ‘listeoajaxlogin’ is causing 502 Bad Gateway error.
If you are not able to resolve it, you’d need to temporary disable ajax login (Listeo Core -> Registration -> Login/Registration Form Type – set to Separate Page) until we figure out the proper fix for that.
If you have any feedback from your hosting provider about that please share with us as it might be helpful for us to came with a fix.
Sorry about any inconvenience caused by that.