iis 7.5 windows server 2008 PHP 5.5 looks like that's the issue I'm assuming letting it be could cause a security issue or issues with sessions?
Seems a long open bug in php (https://bugs.php.net/bug.php?id=64633) affecting Windows 7 installations. The resolution of the microtime() function is reduced resulting in a runtime measurement of 0 in ElggCrypto (based on microtime() taken before and after the very short running benchmark). Because a division by runtime is done in the following the division by zero error occurs.
According to https://github.com/Elgg/Elgg/issues/8603 it shouldn't affect security.
I've created a pull request that should fix this.
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