Player Conversion Error - Could not lookup user uuid for X

Hi,

I was getting my old server from 1.6.4 up to date to 1.16.5 (I guess for 1.17.1 it ought to do/be the same) and I kept getting this error:

[09:39:37] [Server thread/WARN]: Could not lookup user uuid for minecralover[09:39:37] [Server thread/WARN]: Could not lookup user uuid for minecralovercom.mojang.authlib.yggdrasil.ProfileNotFoundException: Server did not find the requested profile at com.mojang.authlib.yggdrasil.YggdrasilGameProfileRepository.findProfilesByNames(YggdrasilGameProfileRepository.java:69) ~[minecraft_server.1.16.5.jar:?] at act.a(SourceFile:53) ~[minecraft_server.1.16.5.jar:?] at act.a(SourceFile:349) ~[minecraft_server.1.16.5.jar:?] at zg.be(SourceFile:530) ~[minecraft_server.1.16.5.jar:?] at zg.d(SourceFile:162) ~[minecraft_server.1.16.5.jar:?] at net.minecraft.server.MinecraftServer.w(SourceFile:645) ~[minecraft_server.1.16.5.jar:?] at net.minecraft.server.MinecraftServer.a(SourceFile:257) ~[minecraft_server.1.16.5.jar:?] at java.lang.Thread.run(Unknown Source) [?:1.8.0_221]

 
Got it for most of the players. And for what I know all the players were actual players, at least I think this server was always run with "online mode" set to "true".

After running the server once I no longer had a "players" folder under my "world" folder and got a "playersdata" folder full with files (same amount I had on the "players" folder) instead, so I'm guessing they got converted. Still, I don't know why I got the error. It also created a folder called "unknownplayers" with some PLAYER.dat files in it. 

Ran the server again and no longer got any issue. Now, are the converted files correct/with players exactly as they were before conversion (I don't know exactly what info is stored in such file)?

Important note: The player data was originally from a Craftbukkit server from 1.6.4, although it was pretty much vanilla in terms of items, etc, so no odd things should be in players inventories, etc. I was trying using a vanilla server jar to convert them because my Spigot/Paper was giving an error as well (same one and another one besides that one), as this was the solution I found people mentioned using, while searching around for this.

Thanks in advance!