The newest patched ETS5 version is 5.7.4 (here), but there is a patch for ETS6 (here), perhaps that works?
Have you tried running the ETS5 in compatibility mode?
If nothing else works, you could send me the .knxproj file (via PN, uploaded somewhere like mega.nz) and i’ll try to import it and/or look inside the file structure if there is some error.