Cannot sign in if multiple sign-in is enabled

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Enable multiple sign-in and sign into multiple Google accounts
2. Go to a Walkaround instance on App Engine (I am using 
wavereactor.appspot.com)
3. Get redirected to https://appengine.google.com/_ah/loginform?state=[long 
string], which will prompt you to select an account (I selected my default 
account)
4. Get redirected to https://[app name].appspot.com/_ah/conflogin which returns 
a 500 error and the following message:

> Error: Server Error
> 
> The server encountered an error and could not complete your request.
> If the problem persists, please report your problem and mention this
> error message and the query that caused it.

What is the expected output?  What do you see instead?

I expected to be redirected to my Walkaround inbox, but was presented with an 
error instead.

What browser and browser version are you using?  On what operating system?

Chrome 18 on Linux

What URL does your browser show when the problem occurs?  Did you compile
walkaround on your machine, or are you using a public instance?

I listed the URLs above.  I am using wavereactor.appspot.com, but I think this 
is an issue with many App Engine apps.

Original issue reported on code.google.com by zmy...@gmail.com on 30 Apr 2012 at 4:32

[GoogleCodeExporter]

Can you provide a more detailed sequence of steps that starts with "sign out of 
all accounts"?  I'm not very familiar with multiple sign-in but I think the 
order in which you sign in to your accounts can have an effect, so it would be 
best to have a recipe that makes it clear what the order is.

Original comment by oh...@google.com on 30 Apr 2012 at 6:12

[GoogleCodeExporter]

1. Sign out of all accounts
2. Sign into primary Google account
3. Sign into other Google accounts by clicking my profile picture on the right 
side of the Google bar and selecting “Add account.”
4. After adding accounts, navigate to wavereactor.appspot.com
5. Get redirected (as stated in the OP) to a page with a list of the signed-in 
accounts
6. Select my primary Google account from the list
7. Get the error shown above

Original comment by zmy...@gmail.com on 1 May 2012 at 12:55

[GoogleCodeExporter]

Not exactly a developer, but I do have some observations.  For starters, I've 
been doing what you describe nearly every time I access my private walkaround 
server with no issues (my experience with walkaround + multi-logins is much 
happier than trying to login to https://appengine.google.com for instance)

As this issue appears to be happening inside your instance there should be a 
log that was generated at https://appengine.google.com describing the exact 
error.  Could you go there, click the "Logs" tab, look for the "/_ah/conflogin 
500", hit the [+] next to it, and copy the error into here?

Original comment by eri...@teamworkgroup.com on 1 May 2012 at 2:17

[GoogleCodeExporter]

see also http://code.google.com/p/googleappengine/issues/detail?id=7144

Original comment by john.douglas.pritchard on 15 Jun 2013 at 1:36

[GoogleCodeExporter]

#3 eric, there is no log entry. The hosting system catches (AFAIK) urls 
starting with underscore and processes them internally.

Original comment by chadmill on 16 Oct 2013 at 1:27