Can't access to OMV admin page (on separate server) after installing Pihole

Hello

Couldn't find a similar problem listed, perhaps it's just a case of a quick tune in ports/blocked lists...?
Anyway, here it is: have a working NAS server with OMV running on an Odroid HC2 since last year; and two weeks ago I've added to my network a PiHole, running on a Pi zero W2 (with usb Ethernet). So it's not the case of OMV and Pihole running on the same machine and address: two different machines on two different addresses.

Both the NAS and the PiHole are working well, but since installation of the latter I can't access the web GUI page of OMV. What I get is:
" This page isn’t working
192.168.1.228 is currently unable to handle this request.
HTTP ERROR 500"

Even though the server works just as before, and I can SSH it just as before.
There's no problem in access the web GUI or SSH in the PiHole.

FWIW, I've added "192.168.1.228" in the whitelist, with no results.

Any clues?

Thanks in advance!

Pi-hole is a filtering DNS resolver.
It blocks resolution of domains into IP addresses, rather than IP addresses.
Adding an IP address doesn't have any effect, i.e. that doesn't block or allow anything.

That's OMV's webserver telling you it failed to serve an HTTP request.
That could happen for a good variety of reasons, where DNS wouldn't be the first suspect.

If Pi-hole would be involved here, then probably for not supplying resolution for local hostnames.

Let's check whether your OMV NAS is using Pi-hole for DNS at all, and whether it would know the name for your OMV IP address.

Run from your OMV machine, what's the output of

nslookup pi.hole
dig -x 192.168.1.228

That's assuming that 192.168.1.228 is indeed your OMV IP address - substitute as required.
Please share the full output, including the exact commands, preferably as text.
(I'll help with the formatting if required.)

Thanks a lot, FWIW a reboot gave no help in this (the whole NAS works with no problems)

Welcome to Armbian 21.08.2 Buster with Linux 5.4.142-odroidxu4

System load:   1%               Up time:       14 days 1:23
Memory usage:  12% of 1.94G     IP:            192.168.1.228
CPU temp:      37°C             Usage of /:    12% of 15G
storage/:      76% of 954G

[ 0 security updates available, 42 updates total: apt upgrade ]
Last check: 2021-11-20 00:00

[ General system configuration (beta): armbian-config ]

Last login: Fri Nov 12 21:58:43 2021 from 192.168.1.251
root@OdroidHC2:~# nslookup pi.hole
Server:         192.168.1.204
Address:        192.168.1.204#53

Name:   pi.hole
Address: 192.168.1.204
Name:   pi.hole
Address: 2a00:23c5:309:ef00:9021:9ac3:4881:201f
root@OdroidHC2:~# dig -x 192.168.1.228

; <<>> DiG 9.11.5-P4-5.1+deb10u5-Debian <<>> -x 192.168.1.228
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 18378
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
;; QUESTION SECTION:
;228.1.168.192.in-addr.arpa.    IN      PTR

;; ANSWER SECTION:
228.1.168.192.in-addr.arpa. 0   IN      PTR     OdroidHC2.lan.

;; Query time: 3 msec
;; SERVER: 192.168.1.204#53(192.168.1.204)
;; WHEN: Sat Nov 20 08:44:23 UTC 2021
;; MSG SIZE  rcvd: 82

root@OdroidHC2:~# sudo reboot
root@OdroidHC2:~#

Your above results show that your NAS is using Pi-hole at 192.168.1.204 for DNS, and that Pi-hole does know OdroidHC2.lan as a hostname for 192.168.1.228.

This would indicate that Pi-hole is operating as expected for now.

Could you share the URL that's prompting a HTTP/500 when trying to access it?

both Chrome or MS Edge can't connect. The latter gives

# This page isn’t working right now

**192.168.1.228** can't currently handle this request.

HTTP ERROR 500

The url can only be the address: 192.168.1.228. It is bookmarked and had always worked, until the PiHole was installed.... How weird!

Since you are using the IP address, this would again preclude Pi-hole from being involved.

Pi-hole could then only contribute to your issue if your OMV would request a domain that Pi-hole cannot resolve.

To verify, watch Pi-hole's Query Log when accessing your OMV's web interface.
Does it show any queries from your OdroidHC2 / 192.168.1.228?
Did Pi-hole block any of them, or return NXDOMAIN as a reply?

Hello, thanks for the suggestion.
Right now, 10:45am, I have tried to access the GUI (IP= 192.168.1.228) but it's not logged:

Time Type Domain Client Status Reply Action
Time Type Domain Client Status Reply Action
2021-11-28 10:45:12 AAAA audio6.mixcloud.com NetworkStreamer-Rpi3B-volumio OK (cache) NODATA (0.3ms)
2021-11-28 10:45:12 A audio6.mixcloud.com NetworkStreamer-Rpi3B-volumio OK (cache) IP (0.4ms)
2021-11-28 10:45:04 AAAA audio6.mixcloud.com NetworkStreamer-Rpi3B-volumio OK (cache) NODATA (0.3ms)
2021-11-28 10:45:04 A audio6.mixcloud.com NetworkStreamer-Rpi3B-volumio OK (cache) IP (0.6ms)
2021-11-28 10:44:56 A play.google.com PC-Desktop-HP8300 OK, sent to resolver2.opendns.com#53 N/A
2021-11-28 10:44:56 AAAA play.google.com PC-Desktop-HP8300 OK, sent to resolver2.opendns.com#53 N/A
2021-11-28 10:44:56 A play.google.com PC-Desktop-HP8300 OK, sent to resolver2.opendns.com#53 N/A
2021-11-28 10:44:56 AAAA play.google.com PC-Desktop-HP8300 OK, sent to resolver2.opendns.com#53 N/A
2021-11-28 10:44:56 AAAA play.google.com PC-Desktop-HP8300 Retried N/A

Looking at the network, the results for the Odroid address go back to earlier on in the morning, while I wasn't using it.
It seems that OMV connects (for various reasons) and is logged, but the web GUI is not even there (query done at 10:46am):

Time Type Domain Client Status Reply Action
Time Type Domain Client Status Reply Action
2021-11-28 08:18:45 AAAA mirror.armbian.de OdroidHC2.lan OK, sent to resolver1.opendns.com#53 N/A Blacklist
2021-11-28 08:18:45 A mirror.armbian.de OdroidHC2.lan OK, sent to resolver1.opendns.com#53 N/A Blacklist
2021-11-28 08:18:44 SRV _http._tcp.mirror.armbian.de OdroidHC2.lan OK, sent to resolver1.opendns.com#53 N/A Blacklist
2021-11-28 08:18:44 AAAA apt.armbian.com OdroidHC2.lan OK, sent to resolver1.opendns.com#53 N/A Blacklist
2021-11-28 08:18:44 A apt.armbian.com OdroidHC2.lan OK, sent to resolver1.opendns.com#53 N/A Blacklist
2021-11-28 08:18:44 AAAA openmediavault-plugin-developers.github.io OdroidHC2.lan OK, sent to resolver1.opendns.com#53 N/A Blacklist
2021-11-28 08:18:44 A openmediavault-plugin-developers.github.io OdroidHC2.lan OK, sent to resolver1.opendns.com#53 N/A Blacklist
2021-11-28 08:18:44 AAAA httpredir.debian.org OdroidHC2.lan OK, sent to resolver1.opendns.com#53 N/A Blacklist
2021-11-28 08:18:44 A httpredir.debian.org OdroidHC2.lan OK, sent to resolver1.opendns.com#53 N/A Blacklist
2021-11-28 08:18:44 AAAA debian.map.fastlydns.net OdroidHC2.lan OK (already forwarded) N/A

I have a similar constellation, Omv on a pi4 and pihole on a pi2. Both works.

Can you Check If your both small computers own a fix IP address in your router?