keep getting disconnected on aternos, and other errors

ok, my first thread here so.
i wanted to fire up my old galcticraft server (with galacticraft addons and other utilies)
however i keep getting "disconnected" or i get a java error saying "timed out" or a "string longer than allowed" error
i get the disconnected error more often and apparentally the server is has a 2 second ping.
i live in scotland btw.
cheers

i also use the MultiMC launcher

biggest bruh, multimc is making my username even longer and i cant change it. anyway thanks for telling me

Quote from Powerful845tiger

Your minecraft username needs to be shorter than 16 characters

I just reaslised the username is 15 characters, am confused

Quote from Powerful845tiger

Share your client log:

1. open your minecraft installation path (enter %appdata%/.minecraft in the file explorer )
2. open the logs folder

3. open the "latest" or "latest.log" file with a text editor

4. copy and paste the content of the file to https://mclo.gs/

5. send the link here

Vanilla server log [#suFr3EO] - mclo.gs

also i made a new xbox account named er9747 and now i just get the disconnected error