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@Tobizon I just switched to Endstone (nukkit fork) and it works fine now

I’m having the same issue, this is my info:

Build ID: 35767063

Branch: hotfix_21_u10

Commit ID: 98aaa32f94e0440ba67be562ccf07fefefbac0d9

I posted about the same bug 4 hours before yours, only to have it marked duplicated. late to the game, sir. I sure wish they have a version filter when searching for issues too, because that's what I find most important when searching in case of a duplicate.

@Ali Nigga I can say without a doubt, at least my case, this is incorrect. In the past I managed a Data Center in Lenoir NC for Google. I can say without a doubt I have everything properly configured to run a simple Minecraft server. No amount of updating a modem that is already running the latest firmware and has all proper configurations will resolve an issue that isn't a hardware issue.

(Once again this is for my personal use case so I can only imagine would be, at least, similar to others' issue.)

@Michael Blaine I can guarantee that the issue is not on the users' end. I have every exception written into every firewall needed for users to connect to the server. I have also checked and verified that the user who *was* having issues has their system setup properly.
I would like to stress "*was*" as randomly and without any change to any system the user can now connect to my vanilla server. No change to their system or to any of mine. This clearly points out to me that it is an issue with the version of io.netty that Mojang is using for the server.
Further more I can verify that the server (logs were checked) was receiving their connection. Correctly authenticated the user and allowed them to join. Only for them to load in properly and then be disconnected moments later with the io.netty error.
In the past week this has happened to two of my users on my server. However since upgrading to 18w32a this has not been an issue. We have made the decision to stick with this version as it is more stable than the full 1.13 release in every way.

I run a small Vanilla server. We recently restarted the whole server since two, out of fourteen, of our users were having this same issue. We attempted all sorts of troubleshooting to find what was causing the issue, to no avail. So I went through this whole process:
• Wiped UNIX Server.
• Switched to a Windows 2016 Server
• Reset and reconfigured Servers Router/Router Firewall
• Reset and reconfigured Servers Firewall
• Configured 3 different routes to the server through 3 different IP's/DNS'
• Started from scratch, no carried over Server Files.
• Switched to 18w30b Snapshot.
I did all of this and everything seemed to work, seemed. The server has been running fine since the launch of 18w30b, about six days I think. Until yesterday, with no change to any settings anywhere a single user, unrelated to the first two before, is no longer able to connect. He has this same error. I have done all the testing I can and still have not been able to find a cause or solution. Nor do I think I have come close to isolating the issue. All I can say is through all the meticulous testing I have done, it is an issue with Minecraft/Mojang and not the end users. 
EDIT: Seems like a potential solution is for Mojang to update the version of netty-all that the server.jar file uses.

I have managed to at least somewhat get my old ascii.png converted. Here are some problems I have found:
• Character spacing must be made at the front of the character. Minecraft currently ignores "Letter Width."
• No matter how 'high resolution' the font Minecraft will down-sample it and make it either look "blurry" or extremely pixelated, this is most prevalent when placing signs.
• There is no good way to convert a .png into a .ttf. I had to re create my entire font pixel by pixel and it still will not come out the same as it once did before 1.13pre-6.
My proposed solutions are as follows:
• Give players the option to disable the new "improved font" as it was stated on the latest patch notes. Personally I am for this option the most since I don't even prefer the new font over the old font. (In reference to default)
• Give players the ability to set the "resolution" of the custom font. Currently I have found that using a program called FontForge has allowed me to seamlessly change the size of my font, however this is not a working solution since Minecraft completely botches any custom .ttf.
• Fix players not being able to use custom resolution ascii.png files. This would be the easiest solution since we know Minecraft can still read from the file, the problem appears to be that Minecraft is unable to properly read higher resolutions ascii.png files.

So far that is all I have to add. I have created 3 different fonts ranging from small to large pixel sizes and have tried up-scaling and down-scaling these fonts in an effort to get Minecraft to read them properly. At this point I can only hope Mojang fixes these bugs because I give up on creating fonts, it has been a lot of work for no payoff.