OK, I seem to have found a solution: I copy to iTunes the files which are in PS Touch on my iPad, and then I extract them from iTunes onto my Mac. I select the PSD format before I copy to iTunes, and I do this operation with batches of approximately 50 files at a time. If I select more files to copy, PS Touch crashes.
Does it help to quit all other applications while you upload?
It would be great if you could add some kind of estimate of the amount of files.
There were 1271 files in my PS Touch library. They were most often multilayered. I copied them to iTunes in batches of 20-50, then saved them to my Mac.
I initially tried quitting all other apps to liberate some RAM, but that didn't help. It must be a memory problem within the PS Touch app itself.