How to diagnose «Something went wrong while saving the category»?

Category isn’t saving with error:
Something went wrong while saving the category.

  1. Steps:
  2. Goto Product - Categories - Underwear: (direct link)
  3. https://www.pumpunderwear.com/admin/catalog/category/edit/id/317/
  4. Upload photo in content category field
  5. Click Save

The «Something went wrong while saving the category» message is shown only by the

\Magento\Catalog\Controller\Adminhtml\Category\Save::execute() method:

As you can see, the method logs its exception, so you should be definitely able to find its report in the exception log file.