I've updated the mod to added some extra things in the ownership check to indicate what exactly went wrong, when it happens again please post the mods' log, hopefully that'll point to something...

This is likely an issue in my code but without replicating it here it's hard to track what causes it in the first place, and it happening after a while makes it hard to debug too xD

Still getting the error. Here is the latest Error Log

[10:48:45] Initialized
GameMode=Survival
OnlineMode=PRIVATE
Server=True
DS=False
Defined=STABLE,
[10:48:45] ### PaintGun v18
[10:48:45] Finding for block skins...
[10:48:45] Defined skin #0 - No Skin ()
[10:48:45] Defined skin #1 - Battered (Battered_Armor)
[10:48:45] Defined skin #2 - Carbon Fibre (CarbonFibre_Armor)
[10:48:45] Defined skin #3 - Clean (Clean_Armor)
[10:48:45] Defined skin #4 - Cow Moo Flage (CowMooFlage_Armor)
[10:48:45] Defined skin #5 - Digital Camouflage (DigitalCamouflage_Armor)
[10:48:45] Defined skin #6 - Disco (Disco_Armor)
[10:48:45] Defined skin #7 - Glamour (Glamour_Armor)
[10:48:45] Defined skin #8 - Golden (Golden_Armor)
[10:48:45] Defined skin #9 - Mossy (Mossy_Armor)
[10:48:45] Defined skin #10 - Rust Non Colorable (RustNonColorable_Armor)
[10:48:45] Defined skin #11 - Rusty (Rusty_Armor)
[10:48:45] Defined skin #12 - Silver (Silver_Armor)
[10:48:45] Defined skin #13 - Wood (Wood_Armor)
[10:49:37] Loaded settings:
ExtraSounds=True
SprayParticles=True
SpraySoundVolume=0.8
SelectColorZigZag=False
HidePaletteWithHUD=True
PaletteScreenPos=0.4345, -0.691
PaletteScale=0.5
PaletteBackgroundOpacity=HUD
AimInfoScreenPos=0.642, -0.22
AimInfoBackgroundOpacity=HUD
PickColorMode-input1=shift c.landinggear
PickColorMode-input2=g.lb g.rb
InstantPickColor-input1=shift c.secondaryaction
InstantPickColor-input2=
ReplaceColorMode-input1=shift c.cubesizemode
ReplaceColorMode-input2=

[10:49:41] PacketJoinSharePalette :: broadcasting (76561198009659686)'s palette.
[10:49:41] + sending all players' palettes back to original sender.
[10:49:41] SkinTestPlayer.Update() :: attempting test number 1...
[10:49:41] SkinTestServer.SpawnGrid() :: spawning...
[10:49:41] SkinTestPlayer.EntityAdded() :: found grid for local player... inScene=True; phys=null
[10:49:41] SkinTestServer.GridSpawned() :: spawned at 34182.79,25934.49,43640.45 and monitoring for paint changes...
[10:49:43] SkinTestServer.Update() :: grid for (76561198009659686) was not painted, yet...
[10:49:47] SkinTestPlayer.Update() :: grid painted
[10:49:51] SkinTestServer.Update() :: grid for (76561198009659686) was found painted, sending results...
[10:49:51] ... sending skin indexes (count=11) = 1, 2, 3, 4, 5, 6, 7, 9, 11, 12, 13
[10:49:51] SkinTestPlayer.GotResults() :: got results! owned=11/13; ids=1, 2, 3, 4, 5, 6, 7, 9, 11, 12, 13
[11:48:59] Failed to apply skin server side, skin not owned.
[11:48:59] Failed to apply skin server side, skin not owned.
[12:41:48] Failed to apply skin server side, skin not owned.

Well it's not in offline mode so the extra debug stuff didn't get shown xD

I've recently had this happen in my world, single player. Adding to the confusion I placed blocks on my grid that didn't show the skin I currently had selected, but placed old default instead. And couldn't change them, even trying different color choice, skin change also. Wondering if there is a conflict of codes in my world. But that's probably a duh thought. Thinking these two might be related.

I'm not sure if anything I added will help, But I hope it does.

As mentioned in the color picker window, vanilla skin selection is different than this mods' skin selection, I can not read nor set the vanilla one due to lack of API.
That means placed blocks are skinned with the vanilla selection, it's annoying I know xD

If your world was in offline mode the extra debug things should be in the mods log, please post it (%appdata%\SpaceEngineers\Storage\500818376.sbm_PaintGun\Info.log)

This is happening for me. Any fixes or anything? Just curious.

I can't fix it without a way to replicate it so I can see what is going wrong, follow the discussion above.

This issue seems to present with family sharing accounts if that helps.

To answer the questions from the below question from @Digi
Seems like you couldn't skin anything on join or something, maybe the server didn't load steam inventory.
That error is more of a warning, if I didn't add that manual check it would've not marked any skin as owned because it had nothing to mark.

Did you join that server as soon as it was started?
Also did you try skinning using vanilla ways?
And did you reconnect later? I suspect it would've worked then if it's the inventory issue.

Can you add the answers to these to the "Paint gun cant add owend skins" topic, would keep it more organized.

1.This was a sever hosted on my local machine in which at the time i was the only one in the game session.
2.I was not repainting anything either with vanilla pain tools nor Paingun
3. As it was a server created my myself upon game start up, the server opened when i created the session thus I did not join later.

To add some extra info, I believe a ship had just appeared via Modular encounters that perhaps caused recolor issues as it was spawning? On top of that unsure if the author of that ship used Paingun to color that ship but I am unsure as to weather spawn ships via modular encounters would retain the code from pain gun while spawning into a world but maybe a possibility. First issue I had after 300+ play hours on this save though.