Unable to reproduce the issue on Win-7 and Win-10 using chrome reported version #60.0.3112.90 and latest canary #62.0.3178.0.

Attached a screen cast for reference.

Following are the steps followed to reproduce the issue.
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1. Opened gmail on a chrome browser and clicked on a label.
2. Observed that label opened as expected and also was able to access the inbox.

Reporter@ - Could you please check this issue on latest canary #62.0.3178.0 by creating a new profile without any apps and extensions and please let us know if the issue still persist or not.

Thanks...!!

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Thank you for the response!

The User has 40,000 + emails stored in < 35 Labels. What happens is she clicks-on a Label to "move" an email into it and the 400 error gets thrown. No Label can be clicked-on-to-open and she has to restart Chrome. Similarly, this happened in Outlook -using GAS to sync with Gmail- with an out-of-memory error, so she started using Gmail exclusively. She started using Gmail about a month ago with no problem until Tuesday, the 1st of August.

I have created a new profile, tried Incognito mode, and termed a  few Labs she had, all to no avail.

I reverted her Chrome back to the previous version 59._._._ last Friday the 4th, August, 2017 and will check with her later today.

I have a case open with Google Chat Support on this; I asked a question: "is there a limit to the number of emails that can be stored in Labels"?; the consensus with all at support there is no, that should not be an issue.

Thank you for providing more feedback. Adding requester "krajshree@chromium.org" to the cc list and removing "Needs-Feedback" label.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Thank you for the response!

User is running 8.1 Pro

The User has 40,000 + emails stored in < 35 Labels. What happens is she clicks-on a Label to "move" an email into it and the 400 error gets thrown. No Label can be clicked-on-to-open and she has to restart Chrome. Similarly, this happened in Outlook -using GAS to sync with Gmail- with an out-of-memory error, so she started using Gmail exclusively. She started using Gmail about a month ago with no problem until Tuesday, the 1st of August.

I have created a new profile, tried Incognito mode, and termed a  few Labs she had, all to no avail.

I reverted her Chrome back to the previous version 59._._._ last Friday the 4th, August, 2017 and will check with her later today.

I have a case open with Google Chat Support on this; I asked a question: "is there a limit to the number of emails that can be stored in Labels"?; the consensus with all at support there is no, that should not be an issue.

It happened again in the previous version 59._._._

Users are allowed Chrome browser only, but I will activate Firefox for her and see if it happens there.

@mholden-- Could you please confirm if this is chrome specific issue or you could see it in other browsers too as per comment #6.

Thanks!

Thanks again,

Way ahead of you as yesterday I allowed Firefox on the suspect computer and it promptly threw the 404 error.

Thanks for looking into this, and I apologize for any inconvenience this caused the Chrome Developer's Team.

I am configuring another computer to try and will let Gmail Support Team know.

Best,

Mike

Thank you for providing more feedback. Adding requester "hdodda@chromium.org" to the cc list and removing "Needs-Feedback" label.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Closing this issue as WontFix as per comment #8. Please feel free to undo if not the case.

Thanks!!!