This would definitely be something server side and / or associated with other software on the server for a few reasons.
While we do use a custom session handler, we use the proper session_set_save_handler and related functions, rather than ini_set.
The only usage of ini_set throughout the software is during an automatic Admin CP upgrade to disable errors, which is a fairly typical use case - and if it fails, the error is suppressed and ignored. This is the case for both 4.16.12.1 as well as the current version.
Based on the error itself, it looks like there may be a configuration issue somewhere in php.ini, or in a .htaccess file that is present on the server, attempting to set that directive (which is incorrect in itself - the directive is session.save_handler instead, though that could be as simple as the server interpreting the dot as an underscore in the error).