Hey Richard,
Is there anything particular regarding your files? When trying to upload 10 pictures with simultaneous uploads set to 10, I encounter no issue. I’m on the latest Discourse version.
Thanks for reporting Richard; this is a tricky one to debug at times. Can you please enable the site setting enable_upload_debug_mode, and hopefully you will get a more detailed error message, and if not if you go to /logs on the site it should show a more detailed message.
I know the UX around this is not ideal either. Currently you can upload 10 files and 3 might fail, but we only show the generic dialog and “that file”. At the very least I can see if I can change that dialog to at least show the file name.
Seeing the same problem here after upgrading to 3.x. Often with anything over 10 files it will cause random files in the batch to error out. DO droplet configured with spaces for uploads. No problems with 2.x and a fairly image intensive forum.