You appear to have removed necessary scripting files, or are using scripting files from a different version of Photoshop (and missing it's necessary support files).
You can probably solve it by copying the entire Presets/Scripting folder from the copy that is working (but make sure to copy any third party scripts you may have installed).
Thanks, Chris. I just had to replace the stacks script folder on the iMac version with the MaxBool Pro content and it worked. It's still a mystery how a file would have been removed on the IMac. Anyway it's working just fine now.
Glad to hear it's all working again.