At least in my configuration:
mediawiki123-1.23.9_1 and mediawiki125-1.25.1 (both tested)
PHP version: mod_php55-5.5.26
FreeBSD 10.1 (10.1-RELEASE #0 r283010)
Running in a jail, the command in MWCryptRand.php

$iv = mcrypt_create_iv( $rem, MCRYPT_DEV_URANDOM );

Times out. if error reporting isn't enabled, a blank screen results.
Modifying that line (320 in 1.23 and 317 in 1.25) to

$iv = mcrypt_create_iv( $rem, MCRYPT_DEV_URANDOM|MCRYPT_RAND );

solves the issue. This seems to be an issue with the availability of DEV file system per this note:
https://secure.php.net/manual/en/function.mcrypt-create-iv.php#117047