Learn more about this change here.
For help regarding Aternos, we recommend you to visit our support center.
Display MoreThe mods' version on your client has to be the same as on your server, not just their compatible Minecraft version.
You can share your client log to check what mods are the problem
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
Logs Latest- https://mclo.gs/6Jtzr3t
There was an error with the assembly, I understand what this error means and reviewed all the mods and they are the same as those on the server and forge is the same, all mods are downloaded from CurseForge specifically for version 1.16.5. Sorry for the mistakes as translated by google translator. 
My logs Minecraft https://mclo.gs/6Jtzr3t , logs server https://mclo.gs/Q24ahcL
