Lots of reports about similar issues, with some variance in the exact error message, resulting in the upload failing:
- https://commons.wikimedia.org/wiki/Commons:Village_pump#Something_wrong_with_the_UploadWizard.3F
- https://commons.wikimedia.org/wiki/Commons:Upload_Wizard_feedback#error_occured_while_uploading
- https://commons.wikimedia.org/wiki/Commons:Upload_help#File_name_not_allowed_.28.E2.80.9EDieser_Dateiname_ist_nicht_erlaubt.22.29
Seems deterministic and reproducible for different users with the same files; the files upload fine with the old interface.
Some of it might be fallout from T86680 but seems unlikely.