I have a Limesurvey server that is running smoothly (Limesurvey 3.25.12+210211) with 1189 active surveys and 213 users.
It is running on a Centos 7.9 machine with MariaDB 10.4.17, nginx 1.16.1 and PHP 7.4.13
Today I have tried to update PHP through yum command (which installs PHP 7.4.15) and after that, no user is able to log in. When a user writes his username and password (I have tries with several users), and pushes the log in button, the login page is recharged again and again.
I have tried with different browsers (Firefox, Chrome and Edge).
I have looked php-fpm and nginx logs and they show no info.
I have also looked the MariaDB log error and it shows no info neither.
I have enabled limesurvey logs following the instructions showed in  manual.limesurvey.org/Optional_settings#Logging_settings , and the only information I get is a short message which says: "[warning] [system.web.CHttpSession] Failed to start session.". I have attached a piece of the log file.
Any ideas? 
Thank you in advance!

Failed to start session is a clear message.

When you upgrade PHP : something happen with session path or something other.

You can use session DB (see config.php) but best can be fic your issue with PHP session.