Cannot post to Mattermost

[bagatel]

Hello,

Using Redmine 5.0.6 and Mattermost 9.0.0 here.
I tried setting the connector as per the documentation, tried to set different values for messenger channel and username (they should be optional for Mattermost I guess), to no avail.
Using the incoming webhook URL with curl just works.

I, [2023-10-03T14:07:01.940753 #1]  INFO -- : Enqueued MessengerDeliverJob (Job ID: bb1ded3b-c129-489f-834b-7b6f48dc453e) to Async(default) with arguments: "https://chat.calenco.com/hooks/XXXXXXXXXX", {:text=>"<https://redmine.calenco.com/projects/test-pm|Test PM> - Ticket <https://redmine.calenco.com/issues/12439|Bug #12439: test 8> créé par *Camille Bégnis*", :link_names=>1, :attachments=>[{:fields=>[{:title=>"Statut", :value=>"Evaluation", :short=>true}, {:title=>"Priorité", :value=>"Normal", :short=>true}]}], :icon_url=>"https://raw.githubusercontent.com/alphanodes/redmine_messenger/master/assets/images/icon.png", :channel=>"Redmine"}
I, [2023-10-03T14:07:01.941250 #1]  INFO -- : Performing MessengerDeliverJob (Job ID: bb1ded3b-c129-489f-834b-7b6f48dc453e) from Async(default) enqueued at 2023-10-03T14:07:01Z with arguments: "https://chat.calenco.com/hooks/XXXXXXXXXX", {:text=>"<https://redmine.calenco.com/projects/test-pm|Test PM> - Ticket <https://redmine.calenco.com/issues/12439|Bug #12439: test 8> créé par *Camille Bégnis*", :link_names=>1, :attachments=>[{:fields=>[{:title=>"Statut", :value=>"Evaluation", :short=>true}, {:title=>"Priorité", :value=>"Normal", :short=>true}]}], :icon_url=>"https://raw.githubusercontent.com/alphanodes/redmine_messenger/master/assets/images/icon.png", :channel=>"Redmine"}
I, [2023-10-03T14:07:01.946706 #1]  INFO -- : Redirected to https://redmine.calenco.com/issues/12439
I, [2023-10-03T14:07:01.948172 #1]  INFO -- : Completed 302 Found in 62ms (ActiveRecord: 19.0ms | Allocations: 21245)
I, [2023-10-03T14:07:02.006093 #1]  INFO -- : Started GET "/issues/12439" for 80.13.182.198 at 2023-10-03 14:07:02 +0000
I, [2023-10-03T14:07:02.006749 #1]  INFO -- : Processing by IssuesController#show as HTML
I, [2023-10-03T14:07:02.006794 #1]  INFO -- :   Parameters: {"id"=>"12439"}
I, [2023-10-03T14:07:02.012186 #1]  INFO -- :   Current user: camille (id=3)
W, [2023-10-03T14:07:02.050082 #1]  WARN -- : #<Net::HTTPNotFound 404 Not Found readbody=true>
I, [2023-10-03T14:07:02.050316 #1]  INFO -- : Performed MessengerDeliverJob (Job ID: bb1ded3b-c129-489f-834b-7b6f48dc453e) from Async(default) in 107.31ms

Thanks for your help!