I got the Dark Matter Pickaxe [W.I.P] with Efficiency V and Fortune III from a quest reward I mined a bit with it and then I logged out. When I logged in my Inventory disappeared and I was at world spawn!

1. it's a work in progress
2. quest reward?
3. without a forge mod loader log I can't do anything

Also, reading this again, kind of sounds like either your player data was deleted or you logged in with a different player account

Happens to me too, but not with EE3. I am not sure what is causing it. Can you give us your mod list?

The player data must have been deleted because I have only 1 account!
And I can't provide the log because I resetup minecraft with a modpack without EE3 because it was causing instabilities!

HQM
EE3 (of course)
AE2
Compact solars
ic2
Chickenbones mods
Enderio
Extra utilities
Botania
Blood magic
Buildcraft
Forestry
Ae2 tech addon
Buables
Factorization
Computercraft
Dense ores
Shadowcraft
Fortune ores
Chisel
Tinkers construct
Vein miner
Inventory tweaks
Iron chests
Mantle
Flat bedrock
Pneumaticcraft

And I think thats it

And ALL mods were latest versions at the time!
I ALWAYS keep every mod up to date!

And yes QUEST REWARD HardCore Questing Mod was updated!

I noticed the same thing. Deleted ee3 and weaponmod.
It was fine for some time and then happened again.
I found something interesting. When I'm logging into minecraft I select singleplayer and the world loads. In this very second I press F11 to go full screen and there I go to the spawnpoint with empty hands.
I fixed it by... not touching F11 when the game is loading.
Now I know: when logging off everything is fine and the player data is saved. I now take a backup of the whole .minecraft dir with a batch file. When I'm logging on and forgot about it and press F11 I exit quickly, delete the whole dir and revert to the backup and it's all good.
I think this is probably a forge thing. I checked the logs and there was 'cannot sync with client' when mc starts and I press F11. Need to report it somewhere but I need more info on this.

I'll try to provoke this error in the afternoon to get clean logs.

TL;DR
It's probably not EE3

Edit: When logging to singleplayer and loading a world (with the help of F11) and the game exits again to the select world screen then you are 100% to get this error while trying to load it second time.

Edit2: I'll cross-check my mod list with Shad0wB1ade list.

The most important. I get this kind of behaviour with a custom 1.7.10 pack with EE3 0.2.295 but as stated before I think it's not EE3.

For me it happened in a dev environment, while having my mod and TE4 installed, but I think it is a Forge issue

Reporting to forge

Closing because without logs no one can work on this. Also, hitting F11 doesn't delete player data.

I think it's not the pressing F11 but the delay the client does going fullscreen (comunicating with the drivers) while in the same time trying to load a world save (the sync error I've seen).