It is difficult to give a reproducible exemple, I summarise here the main components:

We have a front-end/back-end system on an ubuntu 18.04 server. The client side is a web plateform where users can create a project with characteristics (like name, date...). The front-end is coded in javascript, with a mongoDB behind, and communicates with a flask API. The web link is managed by a dedicated web proxy.

We face a very puzzling behavior we don't understand after days of debugging:

The front-end sends a www-form-urlencoded string parameter X to the API. This parameter is optional and can be empty.

We check that this parameter is in the request body

let X = req.body.X? req.body.X : "";

If the parameter is set, the requests succeed. However, if the parameter is omitted a weird behavior happens in the API :

• The HTTP request is received, we can read a few key properties such as requested URL, headers and such
• When accessing the property request.form, a dict containing the parameters as key-value pairs, the API freezes.
• After a short time the requests timesout frontEnd side
• After the timeout, Flask resolves the request with a 400 BAD REQUEST code without raising any exception or calling a set error handler for the 400 code.