Account password auth: help

Hello I am new to Safeline WAF. I am on version 8.8.1.
I am trying to set up an Auth for one of the apps connected to Safeline WAF.
I activated the Account password authetication for the app like this:

Then in Auth > Settings > New user. I set a name and password.
Then I do edit on it and allow it for the app:

The auth challange is now active but if I try to login with my credentials it stays stuck:

I thus checked the access and error logs and I found a 504 on safeline challenge:
xxx.xxx.xxx.xxx - - [01/Jun/2025:19:25:28 +0200] "POST /.safeline/auth/api/auth?sl_site_id=3&sl_check_uuid=90ce64083f594bc3a62098450f6e0b18 HTTP/2.0" 504 14588 "https://my.app.com/" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/136.0.0.0 Safari/537.36"

with this internal error:
xxx.xxx.xxx.xxx - - [01/Jun/2025:19:25:28 +0200] "POST /.safeline/auth/api/auth?sl_site_id=3&sl_check_uuid=90ce64083f594bc3a62098450f6e0b18 HTTP/2.0" 504 14588 "https://my.app.com/" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/136.0.0.0 Safari/537.36"

What's the problem? How can I fix it?
Thanks!