The following error occurred while submitting Git today:

LibreSSL SSL_connect: SSL_ERROR_SYSCALL in connection to github.com:443

Then I tried git clone other repositories, also reporting this error.
The proxy is used, but the solutions given by others, such as restart and reset commands, do not work.
Finally, a solution was found:

vim ~/.gitconfig

Delete the HTTPS configuration and you’re done. The proxy configuration was written to a file, so it’s no wonder Git doesn’t work.
reference
[1]. SSL_connect: SSL_ERROR_SYSCALL in connection to github.com: 443. https://stackoverflow.com/questions/48987512/ssl-connect-ssl-error-syscall-in-connection-to-github-com443