Describe the bug
When running the mod on a server, /skin set upload <path> only properly works once after opening a world. To get it to work again you need to restart the server.

To Reproduce

1. Set up a skins folder in your multiplayer server and upload a .png skin into it.
2. Start the server and type /skin set upload skins/[name].png which will work properly and set your skin to the one from the file.
3. Clear your skin and repeat step 2, and you'll get an error saying "A problem occured when trying to upload the skin".

Logs
The mod hasn't logged anything - not on success nor failure of uploading a skin.

Are you using any other mods?
Yes, some optimization/quality of life mods, specifically: Custom Spawn, Fabric Carpet, Chunk Pregenerator, Discord Link, FerriteCore, Krypton, LazyDFU, Lithium, Overworld Two, Random Patches, Starlight and WorldEdit.

Hi, thanks for reporting. Have you tried uploading the skin file to mineskin.org? Does it work there?

Just tried it out - the same thing happens. It properly works the first time, and on any next attempt it responds with "Malformed url" even though I'm absolutely sure I'm entering the same URL.

It's a mineskin bug then sadly ... I doubt I can do anything about it ... :(

But what about the local file thing? Does it somehow use mineskin?

Yes, it goes through mineskin as well, as it needs Mojang signature.