Hi @Humbledore
Welcome to Q&A forum.
Tag "office-word-itpro" focuses on general issues about Word clients, as your issue is related to Word Online, I will modify the tag. Thanks for your understanding.
Does this issue occur to other Office apps for the web like Excel Online?
I find a related thread "Word Online Error".
As Felix Tao MSFT said in it, please check the time zone on your portal and SharePoint Online site to see if it is the same as the time zone on your PC.
Besides, you could refer to the official document ""Your session has expired" error message in Office Online" to check if this is helpful to you.
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