The problem resides in the pseudo random number generator (PRNG) that LibreSSL relies on to create keys that can't be guessed even when an attacker uses extremely fast computers. When done correctly, the pool of numbers supplied is so vast that the output will almost never be repeated in subsequent requests, and there should be no way for adversaries to accurately predict which numbers are more likely than others to be chosen. Generators that don't produce an extremely large pool of truly random numbers can undermine an otherwise robust encryption scheme. The Dual EC_DRBG influenced by the National Security Agency and used by default in RSA's BSAFE toolkit, for instance, is reportedly so predictable that it can undermine the security of applications that rely on it.
Only a few days old, OpenSSL fork LibreSSL is declared “unsafe for Linux”