I'm using a current version of WordPress hosted at DreamHost (shared server) with the current version of Replace Media Upload and very little else in the way of plugins or customizations.
I've been working my way through a bunch of back posts replacing oversized, undersized, and wrong-color-profile images with better-processed ones, and have been having a heck of a time getting Replace Media Upload to behave like it should.
The behavior is really inconsistent. Some of the time it works perfectly--click "Upload" and a few seconds later, boom, the new image is up. Other times it will sit "contacting" for maybe 30-60 seconds before finally responding and processing the request. And about half the time (literally), it will hang indefinitely and never respond at all (I left it overnight once to make absolutely sure).
It's completely random so far as I can tell; doesn't seem to matter whether I replace just the file or update links, and if I go back and trying again with exactly the same image and options some of the time it will go through almost immediately. When it's working smoothly, it seems like three or four will go through in a row, but when I hit a problem one I might have to try a half dozen times before I finally get it to replace; I'm not sure if this is a problem image, or if it's a problem time period. Smaller replacement images (<100K) seem to help a bit, but that may be placebo effect.
Any suggestions as to what might be causing this, and/or what I could do to resolve it? I absolutely love the plugin, but it's borderline useless to me with this behavior, and driving me nuts on top of it!