My friend and I are having issues where after about an hour in game, it disconnects and says, "Game has gone out sync". We have tried the UnPn and confirmed it's set both at 16000 for us, we have tried uninstalling and re-installing and we both confirmed our internet and PC's have no issue (500gb down, 20up for both of us). I see others playing this so I know it's "possible" to play, but have no idea what still needs to be done for us to play beyond an hour.
Any and all help or tips would be appreciated!

Disable UPnP in your router and in your Voobly settings. Also make sure you have UDP 16000 forwarded, not TCP. Please provide a screenshot of the network diagnostics or paste us the result. If the SWGB Lobby does not have it, run it in one of the AOC lobbies please. Also please take a screenshot of the forwarded port in your router settings, so we can verify the port has been forwarded properly. Thank you.

Hmmmm, so here is my network diagnostics results as well as my router setup that shows I've now DISabled Upnp (which I had enabled before).

I wasn't certain that I did the port forwarding correctly but I found your other thread on this (here, thank you!) and gave it a go. The only one I wasn't certain of was the External port, as my router requires this but on your thread that field wasn't mentioned. How does this look so far? And THANK YOU so much for your response and helping me with this. Giving me hope that this can be fixed has literally made my day!

Port has been forwarded properly in your router settings according to screenshot #2. Network diagnostics looks fine, UPnP has been disabled in your router. Please also disable UPnP in your Voobly settings if you have enabled it (disabled by default) and you should be able to play.

We will give it a try and report back with findings. We'll also confirm our Universal Plug and Play is OFF in the Voobly settings as you suggested. Thanks for your help so far, really appreciate it!

Blast!!! We only made it 30 minutes this time and then we disconnected. Both of us have our settings as I indicated as above. Unpnp off in our router and in Voobly, as well as the port forwarded and the network diagnostics are indicating it should be good.

Any more ideas? We are both using ethernet to connect and have at least 100mp/up and 10mb/down. I've attached a screenshot of the error it shows when the game ends.

Also I had accidentally put the "Default Gateway" in for the port forwarding the first time, and not the IpV4 address. I should have done the IPV4, right?
I tried both and didn't have any more luck. Still disconnecting even faster now at 30 minutes instead of 60 ish. Granted it's right around when the PC gets about 150 people. So it seems to just be at a certain point of units that it disconnects.

Correct, you put the local IPv4 address of your computer into the field.

If you completely disconnect from your game, the issue is not related to port forwarding. When using WiFi please make sure, it's not your WiFi disconnecting randomly. Same goes for your router, it could be possible that you are going offline for a few seconds which would then disconnect you from the game, since it's peer to peer. Try running non-stop pings to see, whether you have timeouts in your internet connection. When on WiFi, please use a wired connection if possible just to test, whether the problem disappears or persists.

We were both using wired internet unfortunately. Neither of our internet's went out either. It seems to always be around a certain point of units in the game. We played on easy and were able to complete a game because the computer only had about 40 total units and we were around the same. However when putting the difficult up to moderate and increasing the population max to 250, the connection always is lost when we get over about 250 total units between the 3 of us.
The game does not close entirely, just shows that disconnect error I had attached before. We're still able to view the post game statistics and when re-launching the game that just disconnected (since it auto-saves) it's only a matter of seconds or minutes before it disconnects again.
It can't just be the game in general because I see others in the lobbies that have games with much more people than us.

It's SO frustrating, I appreciate your help trying to troubleshoot this. I don't know what else we can possibly do.

Another question: are you playing both behind the same router?

They are NOT the same. Both of us have port forwarding on though, and unpnp disabled. I have a "TP Link Archer AX3000" (can see in earlier pictures I posted) and he has the ISP provided (spectrum) router which is an "Askey RAC2V1K". I've attached his port forwarding picture here as well

Okay, a few things we have to figure out first.

Do you only have the problem when playing with your friend?
If yes, does your friend also has the problem when playing with someone else?
If no, please provide the vendor‘s name and model number of your router, as well as the firmware version and date installed.

Thanks.

We have only had this problem playing with each other but neither of us have actually tried to play anyone else as there are only a few other games in the lobby and none of them have been open before. I can try to have one, or both of us, match up against others, but as of now, we haven't been able to. We did play Age of Empires 2 just through the games online system, and didn't have any problem completing that game with similar or more units than we'd had in Galactic Battlegrounds using Voobly.

My router is an AX3000 4-Stream Wi-Fi 6 Router made by TP-Link. My firmware is as follows:

Firmware Version:
1.2.3 Build 20230603 rel.46473(5553)
Hardware Version:
Archer AX55 v1.0

Got it working! I had to download the data patch and NOT be on patch 1.2 and it worked. The discord channel really helped me out. Thanks for your help!

So the problem was related to a patch? Glad to hear you got it working.