Thank you so much!
It was the $:/core/ui/Buttons/cancel that wasn’t working.
I went back in to the un-upgraded Tiddlywiki, got the code from the old $:/core/ui/Buttons/cancel and pasted it in to the new $:/core/ui/Buttons/cancel.
Voila, the button is working!
Old code that I pulled into the upgrade to replace the new:
<$button message="tm-cancel-tiddler" tooltip={{$:/language/Buttons/Cancel/Hint}} aria-label={{$:/language/Buttons/Cancel/Caption}} class=<<tv-config-toolbar-class>>>
<$list filter="[<tv-config-toolbar-icons>match[yes]]">
{{$:/core/images/cancel-button}}
</$list>
<$list filter="[<tv-config-toolbar-text>match[yes]]">
<span class="tc-btn-text"><$text text={{$:/language/Buttons/Cancel/Caption}}/></span>
</$list>
</$button>
New code that had to be swapped out for the old:
\whitespace trim
<$button actions=<<cancel-delete-tiddler-actions "cancel">> tooltip={{$:/language/Buttons/Cancel/Hint}} aria-label={{$:/language/Buttons/Cancel/Caption}} class=<<tv-config-toolbar-class>>>
<$list filter="[<tv-config-toolbar-icons>match[yes]]">
{{$:/core/images/cancel-button}}
</$list>
<$list filter="[<tv-config-toolbar-text>match[yes]]">
<span class="tc-btn-text"><$text text={{$:/language/Buttons/Cancel/Caption}}/></span>
</$list>
</$button>
There was no $:/core/ui/Buttons/cancel in the EditButtons plugin. Don’t know if that means anything.