My issue is related to this thread that has been previously closed
Despite upgrading with the patch provided by @david, I still encounter the issue on one forum out of two (on a multisite install).
https://www.dolibarr.org/forum correctly redirects to https://www.dolibarr.org/forum/
but https://www.dolibarr.fr/forumdoes not redirect to https://www.dolibarr.fr/forum/ and thus I am getting a blank page.
Some other things that I have observed is that on https://www.dolibarr.org/forum/, when opening the main hamburger menu, the FAQ link is now on top of the menu whereas on https://www.dolibarr.fr/forum/, it remains on the bottom (I do not understand why and my understanding is that when updating changes normally applies to all forums on a multisite install).
Thanks, you are right did not think about this but this make sense (another sysadm have added a rewrite rule on the Apache2 proxy side for the working forum ).