[Flask] File upload breaks for files with size >64kb on ARM based apache wsgi servers

[Michael Bayer]

Hi everyone,

I discovered a funny behavior of my flask app, when running on ARM based apache wsgi servers. Could you please give me some hints to debug this? :-)

I have a flask application, that supports file upload.
It runs on an ARM based debian stable server with apache wsgi.
If I upload small files, everything is fine. The problem appears, when I upload files with a file size >64kb. The famous "Bad request - The browser (or proxy) sent a request that this server could not understand." pops up.

Things I tried out so far:
- Deploying my flask app to my productive ARM based webserver (debian stable): Here I discovered the issue.
- Running in flask's development wsgi server on my Development PC: No problem, I can upload files with any size I want.
- Clean debian stable installation in a virtualbox on my Development PC (AMD64) same packages as my productive server, but AMD64 instead of armhf (apache, mod_wsgi): Again no problems at all.
- Clean installation of raspian on a spare ARM Raspberry Pi (based on debian 9) with apache, mod_wsgi: Here comes the problem again!
- Increase apache LogLevel to debug -> no error logged
- Activate FileLogging inside my flask app -> no error logged

Have you seen anything like this?
I unfortunately do not have much experience with web servers/development so I'm stuck with my debugging approaches :/

What could I do besides that?

What information do you need from my side? I can provide any code/configuration you need.

Thank you and have a nice day!
Michael
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