www programming

You can test that if you open a forum's topic at the real url,
https://www.bayonnais.com/board/viewtopic.php?t=13475
the server response return

Failed to load resource: the server responded with a status of 500 ()

before that any output is sent, and before the redirect, that should be done by the JS header code, when is detected that the page is not inside the WP page iframe.

You can also test this, if you do not want to force phpBB to load inside the iframe, and test:
Comment the line, into the overall_header added code so (while you have set to Yes the Recompile Stale Template option into ACP)
then open the forum to the real url, and it will be loaded not inside the iframe.
And you'll get the same error 500, and blank page.
Cannot imagine at moment from where it could come out, but since others pages works, i assume it is a problem you created into template files? (if you changed it).
There is some extension that could interfere?
Overwrite/upload again the file viewtopic.php and the related template file so to assure that it is not corrupted?
Disable and test

I tried everything, including updating phpbb again, replacing the files with new ones, it still doesn't work. I do not understand anything...

I deactivated all my extensions, which I re-activated one by one, until I found the fault. This is Advanced Polls (in version 1.2.4.1). A Beta version is in development, so I will wait for it to be deployed.

Everything is back to normal in my forum.

In any case, thank you for the help and suggestions for avenues to explore, Alessio.

Well done!
To me, if i can say, you should increase the top gap of about +- 70 or 100px or so?
But it is your choice! You have a so big header, so when for example you click into New topic, the page reposition too near the top and you have to scroll to bottom to see the post textarea.
Would be a good idea to add two different gaps values, one to be used for the index, one to be used for others pages, so to have users focused to the wordpress page until they are on index, and so when they start to navigate forums (then when into viewtopic for example) the other gap could be used, so to focus the user to the forum.
May it will be added soon, it is ridiculous easy to do. Just a though, i think it can be a good behavior that we-ll be added soon.

Have a good follow up, if any problem just post! Ciao