Cannot log frontend but backend is OK

Hello

Since a while, some of my users contact me because they' blocked outside, unable to connect at frontend - entering username or email and password doesn't product any effect ; furthermore they cannot get the forgotten password email even after several trials. Usually, I reset their password ton a new value, and they are back in the game.
I tried to switch between CBuilder and Joomla 'remember me' systems, and between CBuilder and Joomla login modules, the same keeps happening to a few users.

Today, I had the surprise to be myself blocked outside to the frontend. I cannot login to the frontend, I cannot get "remember me" email... BUT I can still login to backend.

I didn't see anything in apache logs - but in tthe CBSubs logs I saw this (Antoine GAUDIN is me) at the time I tried ton connect (event if i didn't do anything in CBSubs :
 
 

I have no idea of what happens... any thoughts to explain (and fix) this problem ?
Best regards
Antoine
 

Since you can login to backend your account isn't actually blocked. My best guess is caching. Do you have Joomla's page cache plugin enabled in System > Manage > Plugins? You cannot use full page caching in dynamic environments. It just doesn't work that way as it'll serve a page buffer that may not be the users own since it caches on first load. Same for external services like Cloudflare caching if you're using something like that.

Hello,

Thanks for your answer. Unfortunatelaty, there is no caching enabled on my site - plugin is turned off.
BUT you're right, it' related to navigator caching - if I try from another navigator, or if I clean my cookies, I am back to be able to log in frontend.
Since I'm not the only one but it happens to  several users, any idea on ho to avoid this problem ?

Below, my sessions settings
 

Regards,
Antoine

Do you have any server side caching enabled? Cloudflare for example? Caching is basically the only explanation for your issue.