I recently fired up Konsole and tried using tmux when I suddenly realized that Control-B wasn’t working.
Plasma, in one of their recent updates, seems to have added an “Add Bookmark” global shortcut to all Plasma apps mapped to Control-B. While it’s active, Control-B in Konsole won’t get sent to tmux, but will instead keep adding bookmarks to the Konsole Bookmarks menu.
Open System Settings
Under “Workspace” choose “Shortcuts”
Select “Standard Shortcuts”
Find “Add Bookmark” in the list
Change the Shortcut to Custom, then don’t assign a key sequence. This will set it to none.
Once changed, tmux should immediately start working again