Remove the [UserShortcuts] section in Rosegarden.conf.
Launch rg
Edit > Keyboard Shortcuts…
Filter: play
Select the “Rosegarden main window; Play” row
Double-click on the Shortcut which says “Enter”.
Press the spacebar and wait for a moment.
Confirm the conflict dialog pops up.
Click Ok.
Confirm shortcuts are now: Space, Media Play, Ctrl+Return, Ctrl+Enter
Restart rg
Confirm space is now play.