Dear community, I am new to the 3D Printing world, have been printing for a couple of weeks now and things seem to be working mostly fine, with exception to connecting from my mobile devices to monitor prints.
My printer is a Prusa MK3S which I assembled myself from the Kit (after 10 hours) 
Thanks a lot for your help to address this issue!
What is the problem?
I can connect to my Octoprint server running on Raspberry Pi 4 from Windows 10 without any issues.
But if I try to connect from my iPad or iPhone pointing a browser to the Octoprint IP Address (http://192.168.168.21) it wont work, I also tried with octoprint.local and with 80:80 as ports, no connection, as if the Octoprint server wouldnt exist.
I configured OctoPod on my iPhone and worked fine for some days, now I am getting: "Connection Failed: Service Is down or Incorrect Port".
I tried to use OctoClient App on iOS, I never was able to get a successful connection to the Octoprint server.
What did you already try to solve it?
Verified that I am on the same network, checked network configuration, IP Ranges are the same.
Checked with and without httpS.
Logs (octoprint.log, serial.log or output on terminal tab at a minimum, browser error console if UI issue ... no logs, no support!)
octoprint.log (20.9 KB)
Additional information about your setup (OctoPrint version, OctoPi version, printer, firmware,
OctoPrint 1.4.0 running on OctoPi 0.17.0
Raspberry Pi 4
Windows 10 Version 10.0.18363.836
iPhone 11 Pro (iOS 13.4.1)
iPad Pro 11" (2018) (iOS 13.4.1)
Thanks a lot for your help!
Dear community I found exactly what the problem was. I needed to "Enable Multicast Enhancement" on my Unifi Controller.
I am using 2x Unifi-AP-AC-Pro on my network. To enable this you need to go to: "Settings" > "WiFi" > Edit your Wifi network > uner "Miscellaneous" turn on the "Enable Multicast Enhancement" option.
Your AP´s will restart, reconnect to your network and then everything should work fine.
Hope this helps someone else having the same problem as I, as it took me a while to figure what was going on 
The strange thing is that when I configured the Pi some days back, I was able to use it without many issues from my mobile...anyways, with the above parameter its now fixed.
I have the same issue it was working before. Not sure the root cause. I know I recently updated to 1.5.3. I was on 1.5.0 prior. No time to further troubleshoot. I’ll just send me self a web hook from something else like spaghetti detective. My network gear is google nest. Again my windows can all still connect my iPhones cannot anymore. Cant tell you if it’s the iPhone / google nest / octo print. But I can tell you there is no networking configuration changes on my side no firewall on the Linux side and any window pc can still hit it just not iPhones. I thought it might be safari .. then on iPhone I tried Firefox and chrome all the same. I tried 80 and 443. I thought it could be a browser blocking the self signed cert but the errors are clear that there is no connection not a cert issue. I quadrupled checked I was on the same network with same default gateway on the same subnet, and even tried to do port forwarding on the nest for 443 and 80 just as a quick test. I know the iPhones suck and can only connect on 5ghz if both are available and the pi is on 2.4 but that would make no logical sense to be the issue. To me the most logical sense is the latest octo has some minor hiccup and google nest is being to much of a big brother and updated something. Unfortunately google nest is wayyyy to stupid to end users and doesn’t have options that a simpleton couldn’t understand so the word multicast would never be displayed to the google lemmings. I really wish google didn’t think we were all stupid.