I don't manage to reproduce in XWiki 15.10.10: in particular this step doesn't seem to be possible anymore:
Be sure to have at least a children page with big attachments that prevents it's deletion.
I guess because this limitation has been fixed by XWIKI-8910 or XWIKI-16782. Also main issue was about displaying the error which is probably now fixed thanks to XWIKI-14649 and XWIKI-13052
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