So. We got past the Trend Micro issue and were still having issue with QB 21 shutting down immediately after opening the company file in multi-user mode (my desktop connecting to our company file on our server).

We happened to get help from a top level QB Tech Support person. He had us:

1. Open the QB 21 app

2. On No Company Open screen, Open or Restore an Existing Company

3. Select/open company file

4. At Login screen enter username, enter password

5. Hold the Alt key down immediately when hitting OK

This opens QB with a blank desktop. For some weird reason, suppressing all the other stuff that opens up when QB opens was causing our problems. 

To get so you don't have to hold the Alt key down:

1. Have QB up and running

2. Close down all QB windows so you have a blank desktop

3. Go to Edit, Preferences, Desktop View

 - Multiple Windows (if that's what you like) - Selected

 - Select Save current desktop

 - Deselect Show Home page when opening a company file

This should open up a simple, blank desktop (still has File Edit etc at top and QB shortcuts down the left side (My Company, Income Tracker, etc)

I don't know why this worked, but it did. Our desktops are all 2.4 GHz machines and above with 4 - 6 cores and 8 GB of memory so there shouldn't have been a problem. Our server is new within the last 3-4 months and is a mid-level machine.

I had 6 machines, concurrently running massive reports (Sales by Item Summary for the last 3 years with Display Columns by Day) and then dumping them to Excel. No problems on any of the machines. Our slowest machine was maxing out one of its 4 cores but did not lock up or crash.

Hope this helps someone.