Here you are :
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Debugger entered--Lisp error: (void-variable overlays)
htmlize-sorted-overlays-at(1)
htmlize-faces-at-point()
htmlize-buffer-1()
htmlize-region(1 58)
org-html-htmlize-region-for-paste(1 58)
org-html-fontify-code("int main() {\n printf(\"hello world\\n\");\n return 0;\n}" "c")
org-html-do-format-code("int main() {\n printf(\"hello world\\n\");\n return 0;\n}" "c" nil t nil)
.....
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It is htmlize-faces-at-point(), and only when the exported org-buffer contains a snippet of code. Otherwise, the export process works fine.
Giuseppe Lipari