Thanks for sharing your concerns here in the Community, @erbrel. 

Web browsers collect cache to save time when loading repetitive data and images. Over time these files can become outdated and corrupted, causing issues like the error message you're receiving. With that said, let's try uploading your attachments using a private or incognito browser. That way, we can isolate the issue and, if needed, clear the cache from the browser. 

To do this, you can use any of these keyboard shortcuts: 

• Google Chrome: Ctrl + Shift + N
• Mozilla Firefox: Ctrl + Shift + P
• Safari: Command + Option + P

If you can upload attachments successfully, I recommend Clearing the cache. 

Lastly, you'll want to ensure that your attachments meet the specific requirements. Here are the file types you can attach to customer and vendor profiles, as well as their transactions:

• PDF
• JPEG
• PNG
• DOC
• XLSX
• CSV
• TIFF
• GIF
• XML

You can browse this article for more detailed information: Attachments in QuickBooks Online.

Please let me know if you have any questions or concerns. Take care!