Users on the ffmpeg bug tracker had no idea what to say about this strange error...
https://trac.ffmpeg.org/ticket/7086#comment:7
Even hard-coding the openSSL headers into "ffmpeg/libavformat/tls_openssl.c" had no effect.
The ffmpeg version is the newest git snapshot as of 2018.03.16. The openSSL version is 1.0.1j. Upgrading openSSL is not possible on the current system.
What would your next step be to fix this problem?