itcuchuoi The issue here is that CentOS 7 removed the MD5 encryption support. MD5 was used to enable yealink phones to use the VPN but version 7 no longer admit this method.
For Issabel 2.X it will work without issues. In your case do the following:
1.- Stop the OpenVPN Service
2.- Go to the configuration TAB.
3.- Go to the second step and press the "Clean-all" button.
4.- Enter in the linux shell and run the commands:
cd /usr/share/easy-rsa/2.0/
sed -i 's/md5/sha256/g' *cnf
5.- Return to the EasyVPN conf TAB in your browser and start the process again.
With this you will be able to connect through any client that support sha256 encryption method, but old yealink phones will not work since MD5 is deprecated.