Ensure that your antivirus and firewall aren't blocking TERA by adding the game files to their exceptions.
Check Game Files:
Please download Tera again to ensure that all files are working properly. Some files may have become corrupted and this may disrupt gameplay (game issues). You can download it here: https://tera.novaverso.online/files/
Download and install the WinMRT program, only with this data will the Support Service have all the necessary information.
Download here: http://winmtr.net/WinMTR-v092.zip
In WinMRT, in the Host IP line, enter the IP address from the very bottom line of Best Proxy in the Updater.log text file located in the game folder.