I can't understand why the "reply" button no longer works on the Plone where we used to use plone.app.discussion 3.4.4 (and switching to 3.4.5 does not solve the problem) :
The included/released version for plone.app.discussion in Plone 5.1.6 and 5.17 is 3.0.8 .
The 3.4.X series is used in Plone 5.2.X Off course sometimes core internal packages still keep working if you use a newer release on an older Plone minor release , but this combination is as you can understand not tested and also not supported.
the 3.4.4 may have beeen introduced from the moment I installed collective.discussion.mentioning and I will nevertheless look at this side there too (now that this track seems to me to be explored, since after all, this module uses the same code and it is perhaps since its installation that things may have gone a little wrong ...)