After 'Browsing' an image, it does seem to upload, and the thumbnail appears (inside the Imgur box), but then 'Adding' returns to the text being edited without the image link being inserted. No markup code is generated at all.
Or am I doing something wrong?
The browser is Firefox 89.0, OS: Slackware-current64 (about three weeks old), no visible errors. As I reported, the image does upload, as a thumbnail appears in the upload dialog.
- I just tried to insert the image here (on Meta) and it did succeed.
- I still had the tag open on Stack Overflow, so I tried again there. No luck...
- I reloaded the page on Stack Overflow, and tried again, and yes, it did function properly now. So, ignorant me suspects there's either some way image uploading can get blocked during normal use, or the problem was solved in the meantime. (Note that I did several reloads when I originally detected the issue)