When pre-loading assignments using zip file, there is no indication that the process has started after the zip file is selected.
In our case, the screen remained this way for a considerable time (many minutes). A prompt appeared later (after refreshing) on this page showing progress:
Can there be a progress bar for the entire process? (Uploading zip file; Processing files; Validating data; Creating assignments)
I also found that the progress of verification of data was stuck until I refreshed the page: