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Why am I getting this error when exporting to EPUB (Fixed Layout)?

I've looked for solutions on this forum & elsewhere.
The TOC has been remade from scratch multiple times.
Thinking it could have something to do with footnotes, I manually copied and recreated all of them.
I've tried the IDML trick suggested in another post. Nothing... same error.
The location the error points to appears to be some sort of Cache. Could I just delete it (or move it away)?
Thanks
