Hi @MabTH ,
Thank you for posting in this community.
1.Ensure that the Quick Launch option is enabled. If it’s already enabled, try disabling it, refreshing the pages, and then enabling it again.
2.My guess is that it may be caused by an update incompatibility issue with the browser. Is this a problem with all browsers? Try google or your browser's InPrivate mode to see if the same error occurs?
3.View through another computer or account.
4.Check if the same issue occurs with other SharePoint site collections.
Hope the information can help you. And if there have any unclear or misunderstanding, please feel free to post back and we’ll continue to help you all the time!
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