Some can join, some not! Really weird problem!

Hey guys,

First of all, I am new to these forums, so hello everyone, who is able to help me with this problem.

I am running an 24/7 TeamSpeak server since half a year now and it was working perfectly until
I got a new router, since my modem crashed a while a go. Now I got the Arris Touchstone TG862, an 'all-in-one' device.
Before I had a modem and a router, which was a TP-Link thingy.

Anyway, since I got the Arris router/modem, one half of my friends are able to join my server, the other half is not able. They
get the 'Connection to the server failed' error all the time, the others can join without any troubles (most of the time), including me.

First of all some basics:

TS Client version: 3.18.0.2 (my friends and me)
TS Server version: 3.0.11.4 (64-Bit), had 3.0.10.3 before (did not work aswell)
OS: Windows 10 (1511) 64-Bit
IP: ts3.nolfer4ever.at
internal IP: 192.168.1.2 (of the server)

Things I have already checked/tried:

• turned of my antivirus (360 Total Security) on my server/completely uninstalled it aswell
• reinstalled Windows (changed from Windows 8.1 to Windows 10)
• since Windows firewall was online before and after the router change, I did not try to turn it off
• opened all required ports in my router (pictures #1)
• tried disabling the built-in firewall of my router (did not work) and set it back to original (picture #2)
• using DMZ for my server's internal IP address (picture #3)
• my internet provider (still) allows server hosting at home, so no problem from this side

I think it has something to do with my router's settings, but I am not quiet sure, since it was working perfectly with my old one over months.

Are there any Teamspeak server settings I could have changed wrongly? In case, please tell me, if you need any further screenshots or informations.

Basically, I did not do anything beside changing some permissions on the server.

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Some can join, some not! Really weird problem!