In ui-lib.pl, subroutine ui_table_span calls 'theme_ui_table_hr' instead of 'theme_ui_table_span'.
This would appear to be a simple cut-and-paste error from the previous subroutine, which happens to be ui_table_hr.
The fix has been verified (privately) using the Authentic theme and a module that calls ui_table_span.
Thanks - I'll fix this in the next Webmin release.