Juristis
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Indeed, disabling UPnP helps fix the connection error.
In an attempt to understand why something that should help with the connection actually hinders it, I searched the internet and found information from our English-speaking colleagues about a problem within the game itself; according to them, DirectPlay cannot interact correctly with UPnP. This is likely due to the age of the game, as UPnP wasn't even known when it was created.
In general, feel free to disable this function on your routers if you are experiencing connection problems in the game, especially if the ping in Hamachi works normally. However, it's worth remembering that for some other programs to work correctly in this case, you may need to manually forward ports.
In an attempt to understand why something that should help with the connection actually hinders it, I searched the internet and found information from our English-speaking colleagues about a problem within the game itself; according to them, DirectPlay cannot interact correctly with UPnP. This is likely due to the age of the game, as UPnP wasn't even known when it was created.
In general, feel free to disable this function on your routers if you are experiencing connection problems in the game, especially if the ping in Hamachi works normally. However, it's worth remembering that for some other programs to work correctly in this case, you may need to manually forward ports.