It has been active for about 2 months in a specific forum and has not yet been completed.

I tried to remove and re-add the specific forum but nothing changed.

I have added the link to the client area notes.

Just now, Jirinex said:

I tried to remove and re-add the specific forum but nothing changed.

Could you please clarify what you mean by "remove and re-add" the specific forum?

I removed the forum from the archive list and since the unarchive task ran I added it back to the list.
I have confirmed that it does not work from the forums_archived_post table

10 minutes ago, Jirinex said:

I removed the forum from the archive list and since the unarchive task ran I added it back to the list.

The unarchive task will need to run more than once. It will consecutively run until the topics that need to be unarchived are unarchived. 

it is active more than 2 months without any change.

since then we have updated to the latest version but still nothing has changed

Unarchiving goes from new to old. Keep in mind that the number of posts/topics which are done are done at a rate so that your server does not crash. If you have a lot of topics and a not-so-powerful server, it could indeed take several months to unarchive.

The best way to test if it is unarchiving, is find a topic which is the newest topic which is still archived then run the unarchive task manually, if after a few runs of the tasks, that topic gets unarchived, the task is still functioning. If it is not, please provide us that topic URL and we can look into this further.

I added to the notes a url which in the order you describe should have been archived.
I remind you that the archiving has been going on for 2 months now and since the first days it was largely completed.

The topic there that you have provided is not archived.

It's one of the many that never happened.

I think I've figured out what's going on.
All topics that have been posted directly from a database record have not been archived.

Can you confirm that?

25 minutes ago, Jirinex said:

All topics that have been posted directly from a database record have not been archived.

Can you confirm that?

Please be advised I have tagged this to a developer to confirm.

Sorry for the delay on this. It looks like the dev tag was overlooked on this one.

I can confirm that topics posted by the Pages application are indeed excluded from archiving intentionally. This is due to the comment syncing functionality that exists for Pages.