Made a (long overdue) account to report that I'm getting the same across both linux and Windows devices, Chromium and Firefox, with correctly formatted and not oversized .zip files—both one with an additional mod I was planning to add, and the .zip I've uploaded successfully in past.
I don't see an explicit size limit stated anywhere on mods.skial.com, but, I broke out just my loose files without vpks for posterity. At only 9.6MB zipped, encompassing sound files and a single weapon model; again received the same.