Quick answer
Close the file in Excel (or whichever program has it open) and drop it again. The browser cannot send a file another program holds a lock on, and it reports that as a failed connection. A real network drop is retried twice automatically before AllDone tells you.
When you meet this
Any upload slot, most often a spreadsheet you were looking at when you dragged it in: a fixed asset register, a schedule, a payroll export.
What the messages mean
"The browser could not read '<file>' to send it." AllDone checked the file after the upload failed and the browser could no longer read it. The usual cause is the file being open in Excel, which keeps a lock on an open workbook; a file moved, renamed or overwritten after it was dropped fails the same way. Close it, then drop it again.
"The upload never reached AllDone - the browser could not send the file." No answer came back after three attempts. The usual cause is the same: the file is open in Excel or another program. Close it there and drop it again. Otherwise check the network (a hotspot or VPN that dropped, a proxy that timed out) and try once more.
"The upload was blocked before it reached the app (security layer)." Different again: try once more, and tell us if it repeats; the team is alerted automatically.
What does not cause it
A spreadsheet AllDone cannot read is never reported this way. An unreadable, empty or unrecognised file is stored and goes to Read and confirm, or the slot says plainly what it found ("We can't read <file>", "A report we can't use was sent over"). See A file was not recognised.
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