The problem might be with your maximum packet size. We use the value 512, which should be enough for every network, but who knows.

You can try this command in the command line:

ping www.google.com -f -l 512

You will either get message:

Packet needs to be fragmented but DF set.

or not.

You ...

Using a mod pack assembled and hosted by a friend, I cannot connect to any server.
It doesn't matter if the server is running any mods or not, I don't even get a mismatch error.

When I disable all the mods, I get the appropriate errors (or can connect to unmodded servers)

I have seen similar ...