This is more of a help thread than an introduction thread, though I am sorry to hear you're having issues.
Exporting on Gimp gave me trouble a few times, so just in case-
The few times I made a preset with Gimp and it gave me that error, I just went to the folder where it was saved, and slowly double clicked, as to where I could change its name. Then instead of changing it's name, I manually changed the file type. It gave a quick "This may render the file unusable" message, but I clicked yes anyway. It worked for me. And if it doesn't, you can always just repeat the process to change it back to the previous file type.