I found out that if you have an UploadButton inside a Multi-State Box, change the Box's state to another state and then change it back to the original state, the UploadButton will always return the same error when trying to upload a file:
Error: -1; No File To Upload
Is it possible to send the data from uploadbutton to some variable and, after that, use setfieldvalue?
Its not a bug - Multi-State Box States are cleared once changed, to allow better performance. Which means the files pending upload are cleared.
This comes directly from Wix. Check out the comment here.