I had a similar problem tonight and dreaded doing some of the fixes suggested. My son asked me what I had done previous to the problem and that led to uploading some images to PS C5 and iPhoto. He suggested I quit iMovie and restart iPhoto. Once I did that and restarted iMovie the problem was gone. I will keep you posted if the problem comes back.
Running 10.8.5, iMovie 9.0.9, 2 X 3.2 GHz Quad-Core Intel Xeon, mac pro, 3,1, 26GB memmory, SSD 500GB (system), 3TB movie storage drive plus external eSATA drive for resource movies. Rendering to Quicktime a 2:22 hr. project.
I hit that problem while I was trying to solve the famous error 108 or 49 or 50 when rendering a long project. I tried to cut that long project in smaller pieces, but then I got the not enough memory issue all the time (used to be after one rendering attempt). I moved older projects into a different folder and then I was able to export the movie.
Now I need to be able to finish the rest of the rendering since it seems that some parts of the movie still bring me to the errors listed above.
It's really a shame that Apple doesn't develop their leading iLife applications anymore. After the retirement of iPhoto, what do you think will happen to iMovie?