Well, turns out the problem was Alexsmobs. I don't want to remove it however so the best that can be done is to just wait for the author of that mod to release a fix.
Posts by Vanetavii
Hello, so me, my girlfriend and her brother have been playing on this modded Forge server I setup on Aternos. It was working fine for a few days, until just last night I noticed this problem where whenever I'd start up the server, I'd try to join and it'd either stop me joining, or I'd get on but after a second or two, I'd get disconnected, and the Aternos server would close by itself.
I figured the problem was to do with the Xaero's Minimap mod because I accidently downloaded onto the server version 21.23.1 without paying attention to the fact that the CurseForge page for this mod only allowed 21.23.0, although the server with that mod was working perfectly fine for us all for hours until later that same day. Because of this, we thought the solution would be to remove the 21.23.1 version of the mod from the server and instead install the 21.23.0 one, but then we got this.
The next thing we tried was just removing the mod altogether, but then we'd start receiving another error. (Ignore the one about Biomes o Plenty)
I tried doing so much to fix the problem, I decided to download a ZIP of the world file, set the server to be Vanilla then back to Forge so I could reinstall all the mods and reuploading the world in hopes of seeing that might fix the solution, in which it at least got rid of the log errors about "This/that mod is missing", but I currently now get this error when trying to join the server. (Here's the link to the log from last time I tried launching the server: https://mclo.gs/34ioIQn)
I can't think what else left to do to try fix this problem, but I imagine it's something to do with the files in the world folder itself.
If needed, here is a list of all the mods we had installed. The Windows Explorer folder is the mods I have installed for my Minecraft as well as what I told my girlfriend and her brother to download.
Any help would be appreciated, I can't think what left to do. Thank you for reading!