Causes SSL_CTX_new to return NULL

[青木寛 / AOKI，HIROSHI]

I would like some advice as to why I am getting NULLs returned as a result of calling SSL_CTX_new.

The library I'm using is OpenSSL 1.1.1k.
The argument to SSL_CTX_new is TLS_server_method().
The message retrieved by ERR_get_error and ERR_error_string was the following.
  "error:140A90F2:SSL routines:SSL_CTX_new:unable to load ssl3 md5 routines".
The phenomenon does not always occur, but sometimes it does.

In the environment where the problem occurred, many services were running and memory was scarce, so I suspect that lack of memory was the cause.
Are there any other possible causes?
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Hiroshi Aoki