I don't think the spacer button is related to the spaces between toolbar buttons which is what you seem to be setting in preferences. Nevertheless you are not seeing the double arrows on the right side of the toolbar which on my machine, indicate more buttons are under those double arrows (buttons which will not fit on the toolbar with its current settings).
The spacer button should be decreased when horiz. space is set to a higher value. There's something wrong with auto-width, so the last button is not shown allthough there's enough space where the space button is placed.
This is fixed in Opus 22.214.171.124:[ul][li]...[/li]
[li] Fixed problem with full-width spacers in toolbars when icon spacing set to >3[/li]
[li]...[/li][/ul]Full list of changes here: [Directory Opus 126.96.36.199)