Both things are windows of the System Information app, found in /Applications/Utilities.
Does it open directly from there ?.
System Information.app is listed in the Applications folder on the MBP, but not in the same folder on the iMac,
On the iMac, the list position holds "System Profiler.app" (with the extension hidden).
Get info suggests they are the same application: Same creation date and time, same size, same privileges list.
"System Information" on MBP open on double clicking in Finder (and can be opened from the Apple menu).
"System Profiler" on iMac presents a dialogue box: The application "System Profiler" can't be opened.
Does it work for another User Account ?.
No. Tried on 2 accounts, one "managed" the other an "Administrator"
Identical behaviour in both accounts: Menu choice highlights when mouse pointer is on it. On releasing mouse button, the menu closes. No subsequent action.
Are there any relevant crash or diagnostic logs in Console.app - User Reports for when it fails to open ?. Any log messages at all that correspond with the exact time you try ?.
None. Made several attempts to open from menu, then refreshed the Console. Tried a couple by double clicking on the application in Finder, then closing the dialogue, and refreshed/reloaded Console.
No new log messages in either User diagnostic or All messages.
PS to Eric: Safe Mode and Etrecheck are on my list. Other chores to do before I'll get to them, including some suggestion from Epson on chasing down a printing issue with the same iMac.