Unable to edit users - 500 error

Hello,
I keep getting a 500 error when trying to edit any user.

End of script output before headers: php5.6.cgi, referer: https://equinerising.com/main/wp-admin/user-edit.php?user_id=2&wp_http_referer=%2Fmain%2Fwp-admin%2Fusers.php%3Fupdate%3Dadd%26id%3D9

I switched from FCGI to CGI with no improvement. Please advise how to troubleshoot. Thank you.