Hopeles case?
Err:520 means an unknown formula token and can have various reasons. Without inspecting the actual document in question there’s nothing we could do. Which LibreOffice version btw?
Version is 6. Maybe I send it to you. There is nothing sensitive in it.
Probably best to submit a bug report for this case and attach the document in question. https://bugs.documentfoundation.org/enter_bug.cgi?format=guided
Try Tools/detective/trace error. when done Tools/detective/remove all traces.
Also, if I recall rightly, Tools/Options/LibreOffice Calc/Calculate, Enable regular expressions in formulas. This box is to be unchecked for Excel files. In mine I have another unchecked, No wildcards or regular expressions in formulas. All others are checked. Hope that helps.
Can’t get any help of those. Version is this new 6. Maybe I send part of that file so that errors are there, you willing to check?
But very simple commands like =#NAME? give the error.
Please don’t use the answer field to comment on your case (because it is not an answer to the question), instead either edit your question or add a comment there. Thanks.