One very common action done on a colleague's message is "Reply in Thread", which is offered in the "vertical ellipsis" at the right.
This action should be more prominent - either it should trigger directly when clicking a colleague's message, or it should be first in the menu (at least, above "Start a conversation").
I think it is realy prominent at RC server version 2.4.0 at least:

Separate Reply in Thread button near Reaction button
Aha, thank you very much @ankar84
Our admin upgraded the server this morning and indeed, it is solved now. Sorry, I did not expect the solution would come from the server.
I'm afraid the right margin for messages was not adapted to the chosen solution. The last 2 characters of a long message can overlap the new button. To reproduce, use a long message of short words and send it twice in a row.
I suggest moving these icons left, since there is already a left margin for avatars, and accumulating them on the right will reduce the space available to messages.
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Aha, thank you very much @ankar84
Our admin upgraded the server this morning and indeed, it is solved now. Sorry, I did not expect the solution would come from the server.