I seem to be able to regularly reproduce this issue. Third time I opened up the network request pane in Firefox, and the final three requests had a response of:

{"upload":{"result":"Continue","offset":896532480,"filekey":"1654g2ql859w.y5gs8k.237244.webm"}}
{"upload":{"result":"Poll","stage":"queued","filekey":"1654g2ql859w.y5gs8k.237244.webm"}}
{"upload":{"result":"Poll","stage":"assembling"}}

Happens for me today too: .webm file, 32 MB
.jpg worked well 2 days ago.

PS: After trying the upload the 4th time it works: https://commons.wikimedia.org/wiki/File:Triumph-Adler_SKA_correction_tape.webm

If nothing else, the message used should be switched from apierror-unknownerror to apierror-unknownerror-nocode, although actually useful error would be nice.