A few people have had issues uploading to workshop.
* Make sure you don't have any firewalls/antivirus blocking the SDK & Depth.
* Make sure your SDK editor is in realtime, click in the viewport and press Ctrl + R to toggle it
* Make sure the image isn't too big, people have had some issues uploading due to the image they are using. I would try to make it less than 500kb formatted as a jpeg.
* Other people who have had issues have had success backing up their packages and then reinstalling the SDK. Do make sure to backup your custom packages first though

Sorry for the inconvenience!

Ta, mate!
As soon as I'm home from work this evening I'll try these out!

Status Update:
After trying all of these suggestions I am still unable to successfully upload to the workshop.
I'll keep trying.

Try uploading a different package (maybe a completely fresh template). If you have no issues uploading that then it can help get closer to a solution. I would try to only use A through Z and 1 though 9 in your package and original file names as well. We had one report of someone having issues due to Russian characters - this may extend to other standard english characters as well. If you can upload a new template, you might try changing it one bit at a time to find which file is causing the issue.

Righto. I have tried uploading multiple different packages, but you might be onto something with the names. I'll remake the packages and give it a generic/simple name and see if that works.
I'll keep you posted,
Thanks for the help so far, mate!

Make sure that the source file names that you are importing as well are simple too. Did you have any luck uploading the other packages?

So far I've tried to upload a Tiger and a Mako skin on multiple occassions. It seems my name changes and recreating/renaming the packages has been unsuccesful.

Depth SDK - 11
Me - 0

Nothing your package is contain of should have non-latin symbols in name and filepath (textures/picture/materials and ect.) . Especially if u upload to sdk from desktop. Try to import and then upload from the main hdd path (like e:/)

If all else fails try to down resolution your texture files or change the file type of your textures. The goal of this is to upload a upk that is not too big in mb size.

Coconut makes a good point - if you look at your package in explorer how big is it in MBs?

Oh, uhh gimmie an hour and a half to get off work and I'll double check. It's something around 30 MBs?

My SO has been struggling to upload her skin as well, but she apparently just managed to upload it a couple hours ago. No clue how she managed but I'm jelous as all hell.

30 MBs should be fine, if I recall correctly there is a hard limit on uploads being less than 100MBs.

Yep just checked, its only just under 25 MBs

Okay, so I've exhausted my patience on this, I've tried everything I can think of.
I just had to flat out give my Steam account details to my SO so she could upload it, Depth just kept screaming 'Invalid' at me at every attempt.
Hope a better means of uploading to the Workshop is established in the future.

Cheers for everyones help however!