[JavaScript Error: "Component returned failure code: 0x804b000a (NS_ERROR_MALFORMED_URI) [nsIIOService.newURI]" {file: "chrome://zotero/content/xpcom/storage/webdav.js" line: 205}]
Upload image file with the file dialog or by dragging and dropping images onto the dashed region
Upgrade Storage
See this thread for how you can experiment to see what characters are causing this error, and let us know what you find:
https://forums.zotero.org/discussion/comment/419796/#Comment_419796