File Upload Authentication Error in Appian Portal – "Service account not found"

Hi everyone,

I'm currently facing an issue and I’m not exactly sure what I’m missing. I've tried several things and searched through the Appian Community, but I keep getting the same error.

I have an interface in an Appian Portal where users need to upload a document, and after that, a process should be triggered. However, when I try to upload the document in the portal, I receive the following error:

"File upload authentication error. Service account not found. A service account is required to upload files."

The strange thing is that when I test the same interface in Designer mode, it works perfectly—the file uploads with no error. But in the published portal, this error always appears.

I’ve already configured the security settings on both the constant that holds the folder reference and the folder itself, granting proper write permissions to the Portal Service Account.

Is there anything I might be overlooking?

Any help would be greatly appreciated!

Thanks in advance.