client vpn(mac) said "The VPN process failed to start the port is already in use by another process"

I can see the same error when nothing is occupying the port. So this error message seems to be a red-herring and mask the real error :(

So it turns out that when you use the AWS CLI do download the VPN config file (*.ovpn) the file uses a little endian UTF-16 BOM encoding with non-unix EOL characters. The AWS VPN client chokes on that, and then helpfully gives this "port is already in use" error. I know.

The solution is either to manually change the file encoding, or to d/l the file from the AWS console using the "Download client configuration" button at the top of the VPN endpoint page - this provides a file with ANSI/UTF-8 encoding which works as expected.

I was getting the same error, tried everything I could find on the issue but nothing worked. Eventually I figured out the problem by comparing an older client configuration file (ovpn) with the newer one which was causing this error. Both files were identical except for the last line. The new file included a line "verify-x509-name My Company VPN name". After removing this line, the new profile worked correctly and the connection was successfully established.