403 Forbidden on Ubuntu: Passenger Core SIGABRT / "Directory Index Forbidden" in PUN logs

I saw in this discussion:
https://discourse.openondemand.org/t/403-forbidden-error/3635
that the issue was caused by the open file limit on the system being too low.

I believe I might be experiencing the same problem.

See below:

enics@enics-h100:/etc/systemd/system/apache2.service.d$ pid=$(sudo cat /var/run/ondemand-nginx/testuser/passenger.pid)
enics@enics-h100:/etc/systemd/system/apache2.service.d$ cat /proc/$pid/limits | grep "Max open files"
Max open files            1024                 1048576              files

I’m not sure which commands I should run to resolve this.
Any guidance or assistance would be greatly appreciated.

Thanks.