When used with Thermos or Svarka (A 1.10.2 Forge + Bukkit server), LP loads without issues. But, when logging in, this is what happens.
https://pastebin.com/bq6XjUNT
Don't worry I'll report it to Svarka too.

PS : I know Sponge exists and is "better". The problem is that Sponge is not optimized for small servers, as being a coremod. This kind of "hacks", however, are way more efficient and stable (when finished). So I would really appreciate if you could help solving this issue if that is your fault / your are able to fix it.

Need your full log file please

Here they are :
Thermos : https://pastebin.com/XXzjJJQi
Svarka : https://pastebin.com/NEpRzeTG

You need to use the legacy jar on 1.7 servers

https://ci.lucko.me/job/LuckPermsLegacy/

And on the Svarka 1.10.2 one ? LuckPermsLegacy does not work on it.
No more errors on the 1.7.10 one, but the client can't connect...

[07:33:52 INFO]: Done (6.566s)! For help, type "help" or "?"
[07:33:52 INFO]: Registered command forge with permission node net.minecraftforge.server.command.ForgeCommand
[07:33:52 INFO]: Hey, you don't have a server icon! Try out one of Thermos' Icon Generators at https://github.com/CyberdyneCC/ThermosIconGenerators !
[07:33:52 INFO]: [Thermos] Refusing to lock affinity, disabled in cauldron.yml
[07:33:52 INFO]: New version of Thermos available: 1.7.10-1614.57
[07:33:52 INFO]: Download at: https://github.com/CyberdyneCC/Thermos/releases
[07:34:12 INFO]: UUID of player Aang23 is a948d31e-e8ec-4c91-a4f0-e5ebf81d2274
[07:34:27 INFO]: Aang23 lost connection: Disconnected
[07:34:28 INFO]: UUID of player Aang23 is a948d31e-e8ec-4c91-a4f0-e5ebf81d2274
[07:36:09 INFO]: Stopping server

No reason to believe LP is causing the issue on Thermos now.

I can't see any trace of LP enabling in your Svarka logs.

That normal those outputs were in the terminal.
https://pastebin.com/LEBBRX9T

No clue then, nothing there suggests why it's broken

Even if it loads, when a player try to join, the error is the same as in the first pastebin link.

No clue then, nothing there suggests why it's broken

I need some sort of error message to be able to help

I know... I'll be back in 20 mins and I will try to see what I can do to get one... A good old debugger may help.

Here are some more logs that may help.
https://pastebin.com/mYD2QLwb

Seems like the AsyncPreLogin event isn't being called. Is your server running in offline mode?

Either way, this is a bug with your server implementation, not LuckPerms.

No, it is running in online mode.
I'm informing the dev of this bug, and i'll try to fix this on my side.

Ok awesome :)