upgrading from 1.5.1 to 1.6.0 causes invalid XLL file

i was upgrading a project from 1.5.1 to 1.6.0 but Excel gave me the (in)famous "ActiveX component can't create object" error. upon investigating i unzipped/extracted the EXCELDNA file from the XLL file.
in 1.5.1 that is also a zip compressed file (containing folder ".rsrc" and files ".reloc" and ".text"), but in 1.6.0 it is not a zip compressed file, but i don't what it is. opening the XLL in Excel will give an unsupported format error.

file attached; [EXCELDNA (renamed to EXCELDNA.zip for upload policy reasons)](https://github.com/Excel-DNA/ExcelDna/files/10870904/EXCELDNA.zip)

any idea(s)?