Callback failed: server did not respond

Not sure if others have seen this behavior, but I thought I would comment about my recent experience w/ this error:

callback failed server did not respond

In my case, I have never seen this error in 2+ years developing/improving my app. When I noticed this error yesterday, most packages were up to the latest version, with the exception of Dash, Flask, and Werkzeug but they were only a minor version or two behind.

So, I updated to the following versions:
Dash: 2.3.1
Flask: 2.1.1
`Werkzeug: 2.1.1

After upgrading to these versions, and upon app load, I would receive the above mentioend error. I could make it go away after refreshing brower via F5, but functionality of the app was degraded, and I freqently received this error again during app usage.

As a test, I downgraded the above packages to the following versions (the version they were pre-upgrade):
Dash: 2.3.0
Flask: 2.0.3
Werkzeug: 2.0.3

After downgrading, the error went away. I haven’t had time yet to dig into this deeper, but wanted to post my experience should anybody else be experiencing this issue.


I’m experiencing this same issue on Dash: 2.3.1. I’ll let you know if I stumble across a solution.

We’ve started seeing some reports of this here and there. We believe that this bug is occurring from a transfer encoding bug in Werkzeug’s latest release: invalid Transfer-Encoding header for 1xx and 204 responses · Issue #2375 · pallets/werkzeug · GitHub. This has been fixed on their side but not yet released.

We believe you can downgrade werkzeug until this fixed to 2.0.3: pip install dash werkzeug==2.0.3.

Their changelog is here: werkzeug/CHANGES.rst at main · pallets/werkzeug · GitHub. Currently 2.1.2 is tagged with “Unreleased”:

Once it becomes released, we hope that this issue will be resolved.


@bigmike @chriddyp Werkzeug 2.1.2 released today! Upgrading this along with Dash and Flask to latest versions seems to be w/o problems.