Friend can't join his or my server

So this particular issue is confusing me:
We're using the Horizon 3 pack, and when my friend started up a server, I was able to join but he wasn't.
When I tried hosting the server, we had the same issue.
Whenever he tried to join, it would say in-game that he left (but it would never say he joined first).
He's using the right version of Horizon 3, and he was able to join other non-Aternos servers with the modpack.
I'm not really sure what's happening here, and would appreciate the help.
Thanks in advance.

According to the log, he immediately loses connection and times out the moment he joins.
https://mclo.gs/a09ZZkd

Some things I forgot to mention:
From my friend's point of view, he just takes forever to log in, and it continues logging in even after it tells me he's left. If he lets it log in for a long time, it eventually just disconnects.
My friend has tried reinstalling the modpack, and we're certain the modpack isn't the problem, as I was able to get into both of our servers with no trouble.

remove that what

I apologize, I'm not very adept in fixing these sorts of things.
Really sorry about this.

Thanks for the instructions, I had no idea what I was doing.
I looked into the playerdata folder, but there's no file with my friend's name in the title.
There's one with my name, and then a backup of it.
There are 3 other files with a strange mixture of numbers and symbols- I don't suppose that's my friend's file?

Again, really sorry for all the trouble.

(Sorry for late reply, I was away)

Using the website, I now know my friend's UUID.
However, it doesn't match the three files I mentioned earlier. The three files all have the same string of numbers. It's worth mentioning that my playerdata file was an "INVO" file and a "BACKUP" file. These three, however, are "ASTRAL", "INVO" and "DAT".
I did some looking around, at the UUID of these three files also appears in the "stats" folder. It's the only file in there.

Should I consider enabling cracked for future servers, or might this bring unwanted features?

Uh, I don't suppose anyone's still willing to help me solve this?

All that information is correct.
He does not live in the USA.

I think its important to state again: This also happened when he hosted a server. I was able to join easily, but he would have the same issue as when I hosted it.

That's the thing:
We can't actually connect using the regular IP.
For some reason, it never works for either of us.
Instead, he and I both use the DynIP (which normally works for us apart from this problem).

He hosted a server via Aternos.
When he did so, I was able to join with no trouble. However, he couldn't join his own server for the same reason he couldn't join mine.

He is able to open it without any difficulty.