i also had big problems until i got it work (migrated from mqtt1 to mqtt2,4). as already mentioned i think it is important not only to install the mqtt binding, but even the “embedded mqtt broker” (to be found under misc). but then you must not have an external mqtt-client installed because they will not work parallel.
for me this was helpful:
and
sorry when i cannot help more, i´m a pure beginner and lucky because today got mqtt2.4 working finally
ok, so i guess it really was as simple as just deleting the “old” mosquito broker.
Did that yesterday evening and all of a sudden this afternoon i saw my embedded broker online.
Since then i even got one of my tasmotized sonoffs working via openHAB