First: Yesterday i've pulled the latest git repo. So maybe it's a new bug.
Problem: Since today, all login attempts with an existing user account get an "502 Proxy Error" in an alert-box with this content:
Failed login attempt, please try again.
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"> <html><head> <title>502 Proxy Error</title> </head><body> <h1>Proxy Error</h1> <p>The proxy server received an invalid
response from an upstream server.<br />
The proxy server could not handle the request <em><a href="/login">POST /login</a></em>.<p> Reason: <strong>Error reading from remote server</strong></p></p> <hr> <address>Apache/2.2.15 (CentOS) Server at blahblahblah.com Port 80</address> </body></html>
Amazing: When you register a new account; or when you've logged in yesterday and didn't logout, then you can use the whole forum and every seems to be fine.
Setup:
nodeBB latest (pulled from git repo)
redis 2.6.10 (Shared host)
Apache 2.2.15 (Shared host)
.htaccess: RewriteRule ^(.*) http://localhost:4567/$1 [P]
Check your logs for errors.
Source: https://forums.openshift.com/frequent-502-proxy-errors
redis and nodebb logs are fine.
the error appears immediately.
no lack of diskspace.
In the meantime i've installed nodebb from the scratch on new machine.
The same problem appears also with a fresh installation. 
You can reproduce it so please report an issue to the Bug Tracker.
First I try to find the bad commit.
You can try a new install with older NodeBB core files.
It can be related to this issue.
https://github.com/NodeBB/NodeBB/issues/3029
I got it. When i checkout commit ceba856006a549d39d4c5d7c9b3363fb03cac2ae everything is fine. But with commit fa9fde43fb1e3d9a67000b7f69261d8eb150f3d5 it doesn't work anymore.
Have you updated theme files?
Yes.
(filltext, filltext)
Here on community.nodebb.org you can send as many confirmation emails as you want. It means it wasn't updated with this commit.
I don't know. Just when i checkout the previous commit from Julian, it works again.
It had nothing to do with confirmation-emails. I could trackdown to this file src/database/redis/main.js
Yes, I saw your comment on GitHub. Great job, thanks for your time and input! 
It drove me crazy. 
This is fixed on master, thanks for reporting.
