Could you post a screenshot ? What is written in the Journal logs ?
Journal looks fine. After clicking OK, the terminal shutdown.
My guess is you're running out of resources. 20 terminals, depending on what each one is consuming... not to be sniffed at. Try looking into that.
(I know Alain loves guessing)
I looked at some threads in MQL4 forum and found out that MT4 also has the same problem.
I've tried installing terminals on higher spec PC, but I can still open the same amount of terminals, so I guess this is not the problem with resources.MT4 terminal limit is 32 terminals. I guess MT5 limit is not significantly less. Still, I cannot attain the limit.
Out of interest
How many charts are you running per terminal?
How many indicators / EAs?
Max bars on chart?
What are you seeing in Windows Resource Monitor when you open all the terminals?
Anything unusual going on with the temp folders?
Have you looked in the system logbook (EVENTVWR) to see if there is an application error code?
If there is any then you can use the error code and Google to find out what or why it happened.