Error Database Connection

Currently I'm developing a project using:

• PHP Framework CodeIgniter (V.2..)
  
• Database SQL Server 2018
  

Actually this is my old project. As usual it runs well, but sometimes if I refresh it many times or there are about 10 users accessing it in real time, suddenly I get error although my database setting is correct. The error says:
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A Database Error Occurred
Unable to connect to your database server using the provided settings.
Filename: E:\wamp\www\dev_Clin4\framework\system\database\DB_driver.php
Line Number: 125
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I use XAMPP version 2.5.8 as my local server and SQL Server as the database. I'm just curious, it's a framework issue; can't handle a huge database so the database server disconnected it automatically, perhaps the query is not effective, bad scripting or maybe...

Maybe anyone can share the experiences due to this issue or solution?
Thank you