Problems with your system email can prevent you from sending email to clients or connecting to your mail provider.
Issues in the Client Area or your online store can hurt customers' ability to purchase your products or access their purchased services.
WHMCS uses a MySQL® database to store all of your business data, including client and financial records.
Problems in the support ticket system and its related features can prevent email replies from importing properly and cause other disruptions.
If the system cron configuration is incorrect or it encounters problems when running, you may see problems with automation throughout WHMCS.
Make sure to check for any problems both during and immediately after the upgrade process.
While developing your own custom code, you may need to test and troubleshoot your implementation of the WHMCS hook system.