But still: Windows resource manager shows that RW is using approximately 400-500 MB of the main memory when the exception occurs, leaving several GB of free memory. So the image size and number of tabs should not make a difference even if it is a 32 bit application.
Windows often lies about stuff like that.

And the other mystery remains: at startup after the crash RW comes up with exactly the same number of tabs and images open. The only difference is that the offending action that led to the exception now runs smoothly.
That strongly implies memory fragmentation. When you start the product anew, all the fragmentation that occurred during the previous run is reset. So the problem doesn't arise after the restart, but it would very likely occur after further use of the product for some amount of time, during which memory fragmentation would again arise.