Some more details that may shed some light.
My server is an NGINX/Apache hybrid managed by Runcloud. I've been running two different OSTIckets on these servers with no issues. Perhaps v1.16.5 has some differences that don't play nicely with NGINX in some way?
Running PHP 7.4.32, PHP 8.0.24
Disabled PHP extensions:
getmyuid,passthru,leak,listen,diskfreespace,tmpfile,link,ignore_user_abort,shell_exec,dl,set_time_limit,exec,system,highlight_file,source,show_source,fpassthru,virtual,posix_ctermid,posix_getcwd,posix_getegid,posix_geteuid,posix_getgid,posix_getgrgid,posix_getgrnam,posix_getgroups,posix_getlogin,posix_getpgid,posix_getpgrp,posix_getpid,posix,_getppid,posix_getpwuid,posix_getrlimit,posix_getsid,posix_getuid,posix_isatty,posix_kill,posix_mkfifo,posix_setegid,posix_seteuid,posix_setgid,posix_setpgid,posix_setsid,posix_setuid,posix_times,posix_ttyname,posix_uname,proc_open,proc_close,proc_nice,proc_terminate,escapeshellcmd,ini_alter,popen,pcntl_exec,socket_accept,socket_bind,socket_clear_error,socket_close,socket_connect,symlink,posix_geteuid,ini_alter,socket_listen,socket_create_listen,socket_read,socket_create_pair,stream_socket_server