Error when creating project (Convert to target, etc.) . "Value cannot be null. Parameter name: key"

Question

Using SDL Trados Studio 2021 SR1 - 16.1.3.4096 
 Creating a project in the usual way, I get an error I've never seen before. Repeat client, nothing obviously unusual about the source file. docx. I did look at the knowledge base and saw a suggested fix relating to removing endnotes for 2021 SR1 but that's not the issue here. 
 I am working around it by pasting text only into a file and getting the translation done. But if you know of any workarounds to get the file to import, I'd be delighted. 
 Karen Tkaczyk

Karen Tkaczyk · Accepted Answer

Update after the fact: I found an endnote after all, so that might have been the cause.

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Error when creating project (Convert to target, etc.) . "Value cannot be null. Parameter name: key"

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