CAUSE:
This problem can sometimes be caused by running out of system resources. The SperrySoftwarelog.txt file will sometimes contain error messages like "Out of memory or system resources. Close some windows or programs and try again."  It may also contain error messages that state "Error processing type Nothing and Nothing."

RESOLUTION:
It may be beneficial to copy the items that are causing the problem from the original folder into (at least) two subfolders and operate on the items there. To do this, it may be helpful to copy half the items into say, a subfolder A, then operate the add-in on that half. Then create another subfolder, B, and copy the rest of the items into that subfolder and operate on it. Check the contents of the two subfolders, and then go ahead and empty the original folder completely. Finally, "pour" the items of both halves back into the original folder and operate the add-in one more time on the newly combined folder. Be sure to make backup copies before starting.