These core dump files are generated by the operating system when one of the processes running in memory does not finish as it should (crashed), investigating these files should reveal more information about the crashed process and the reason for that, if you are on a hosting plan that provide shell access, you can run any of these commands:
gdb -c core.5817
strings core.5817
Which I assume your developers already did, I would appreciate if you can share with us what exactly is related to Wordfence in these core dump files, also you can go to (Wordfence > Tools > Diagnostics) and scroll down the page till “Send Report by Email” and send the report to “alaa [at] wordfence [dot] com”, make sure to include your forum username, I will take a look at this report to see if there is anything unusual with your server setup.
Thanks.