Theme logo won't display

The logo from an earlier version of the site had disappeared. It is not square and is too small. I prepared a larger, square logo with side 536 pixels, but received an error message (“An error occurred in the upload”) either when I use drag-and-drop or “Select Files”. Furthermore if I choose the Media Library tab, the progress icon spins endlessly and no files are displayed.

steps: Customize, Site identity, Select image
version: WordPress 4.4
Theme: Thematic

The file does actually get up loaded. I can view the Media Library. When I click on the “Attach” link, I also get an error message.

I’d much appreciate any advice on trouble-shooting.