It's not hard-coded. It's passed through filter hook:

$max = apply_filters( 'wpforo_max_logged_topics', 100 );

I assume you know how WordPress hooks work and how easy to change the values passed through filter hooks: https://www.wpbeginner.com/glossary/hooks/

As we suspected it was definitely connected to a certain amount of messages in our forum. We finally found out that you hardcoded in wpf-includes/class-logs.php to 100 the number of messages which are loggued and marked as already read...

If you really need to fix the maximum number of logged posts, this "feature" should be defined in some configuration file and not directly hardcoded in the PHP class file in my opinion.

The read/unread data initiation comes from the cookies. wpForo has wpf_read_forums and wpf_read_topics cookie variables, they contain data of current user, so if the cookies are conflicted and reset then the read/unread tracking will be stopped.

Also, you should make sure the wpForo Cookies are not disabled in Dashboard > Forums > Tools > Privacy and Rules tab.

I think this is a cookie conflict. Some plugin manipulates with cookies and affects wpForo cookies. It's hard to register in your website, so I have to ask you to register a simple user and leave the login details here. I've set this topic as private.

I enabled the footer link => "Mark all read"

When we click, no problem all topics are set as read, but when someone add a new post in any topic, the number of "posts read" is reset, even though just before clicking on the "Mark all read" we were reading this topic

Sometimes, when we read a new topic or a topic never read before, another topic get its "number of posts read" reset

As we have thousand topics, is there any limit of number of topics with posts read ? In the cache ? In the database ?

Thanks in advance

Again, I'd ask you to enable the wpForo footer and click the button.

Also, if we cannot see the issue in the test website, then please provide with the live website URL.

We already took care of removing the forum from our wordpress cache system, so there is no issue related with other cache system messing with the files.

But we still experience our issue.

As mentioned before this isn't happening on our test forum where we have few topics, but only on the production one where we have thousands of topics. And since this issue doesn't seem to happen on some other forums we were wondering if it could be related to the huge amount of topics we have here. 

Correct me if I'm wrong but actually your cache data in wp-contents/uploads/wp-foro/cache/item/ where you store serialized array of data. I am not aware of any limitation on serialization but maybe I'm missing something here which could cause the array to be truncated before being serialized?

For now, we have two types of cache, Cache from a plugin for WordPress and wpForo built in cache.

WordPress cache plugins can't cache wpForo forum page, because wpForo is not based on WordPress Custom Post Types and those plugins have no idea what they should cache and what not.

These plugins cache the whole forum page and always display the same content. You'll not be able to see new topics and posts information. And all other information on forum page cannot be updated, because of cached/static forum page. You'll always see outdated and wrong information. 

But wpForo built-in cache system does a very well cache for your forum, so it doesn't need other cache plugins, and it works as you mentioned above.

I can confirm we have wiped the cache and excluded the forum from our cache plugin so the problem doesn't lie there.

In our forum we have tons of threads and tons of messages in each. Could it be that when you create/update the cache file for each user there is some overflow that crops the array of data loaded into the cache file?