Submitting to shutterstock has become a big pain the but. Often I encounter a message saying that "an unknown error" has occurred but there is no indication of what the error is and how to fix it. An entire batch 20 images that won't submit and I don't get paid enough to dwell on it much further. Anyone else?

Since they updated the site it's become really glitchy, and I often have similar problems. Press the save button, rather than submit, then switch to the legacy site to finish the submission, as it usually shows you what the problem is.

Submitting to shutterstock has become a big pain the but. Often I encounter a message saying that "an unknown error" has occurred but there is no indication of what the error is and how to fix it. An entire batch 20 images that won't submit and I don't get paid enough to dwell on it much further. Anyone else?

Computer type? Browser? Operating System? Without that, no one can answer.

For example, I'm using the new system (which I'm no fan) Firefox and Windows 10. No problems.

Have you tried this?  https://support.submit.shutterstock.com/s/article/How-do-I-upload-content-via-FTPS?language=en_US

Some people have problems because of their Internet connection. Is it shared, is it roaming? Are you WiFi or wired or Optical. What's your network S/N ratio.

Last of all WAG, how big are the files? I've had problems on some sites, with really large versions. If I open and save at 90%, then try again, they fly through.

Good Luck