1
«Redis session exceeded concurrent connections» is a defect of Magento 2.4.8.
2
There are exactly 2 unique results on Google for this diagnostic message:
2.1
The official Magento issue tracker: github.com/magento/magento2/issues/38851
The defect is marked as resolved there, but in reality, users in the latest comments indicate that this is not true: github.com/magento/magento2/issues/38851#issuecomment-3768782189
2.2
The Release Notes for Magento 2.4.8
experienceleague.adobe.com/en/docs/commerce-operations/release/notes/magento-open-source/2-4-8
It just reprints the incorrect statement from the official issue tracker that the defect has been resolved.
3.
I have currently resolved the problem by switching the session storage from Redis to the file system.
I dislike such solutions.
For many years, I worked exclusively on hourly projects, where I had the opportunity to properly resolve Magento defects that no one else had resolved before, becoming the first person in the world to do so.
However, in a fixed-budget project, this approach leads to bankruptcy.
If storing sessions in Redis is important to you, I can fix this problem for you before anyone else in the world, but in a separate project.