You have 2 index files:

index.php = without error

index.html = with error, delete this

------ Little tutorial ------

WebSite X5 > 5 Export > Export the website to the Internet

Click on the symbol on the right for "Target directory", a window opens, mark the index.html file here, then click on the small red X above it and the index.html file is deleted.

Now when calling the web address, the file index.php should be displayed without the error.