After Migration, Warning: include_once() errors on all pages

This is common issue when migrating from one host to another. You have those errors because path to include file is not correct (path do not exist on new host). Maybe you used absolute paths to files on old host, now, on new host path is different and that is why you have those errors . OR, those files are not moved correctly from old host to new host.

What you should do is next:

Firstly, check if mentioned file(s) exist on new host (for example this file “includes/class-wc-download-handler.php”), if not, transfer it from old host again to be sure.

If file are there, and it is correct size, than open “/wp-content/plugins/woocommerce/woocommerce.php” go to line 258, or search for “class-wc-download-handler.php” check and correct if needed path.