Can't access Open Media Vault web interface over internet : Invalid IP Address

I'm trying to access my OMV web interface over the internet. I have another Raspberry Pi with a webserver on it (NextCloud) on my network so I've had to change the port of OMV to 81 and I port forwarded accordingly. I can login locally just fine. I have a domain and subdomain already setup, and I can access the login screen via mydomain.com:81, but when I login, I get this error:

Error #0:
OMV\HttpErrorException: Invalid IP address. in /usr/share/php/openmediavault/session.inc:185
Stack trace:
#0 /usr/share/php/openmediavault/session.inc(207): OMV\Session->validateIpAddress()
#1 /var/www/openmediavault/index.php(34): OMV\Session->validate()
#2 {main}

Any ideas? Do I need to allow this IP address or something?
Cheers,
Josh