Increasing SQL Timeout value broke CRM, with a plug-in execution failed error

Recently we ran into this error on Dynamics 365 On-Premise.

The plug-in execution failed because the Sandbox Client encountered an error during initialization

The cause of the error was because we played with the registry and web.config on the CRM server. The requirement was to increase the time limit because deleting the managed solution would cause an 'SQL Timeout' error.

We have followed the MS article to increase the Sql Timeout value:
https://support.microsoft.com/en-us/help/918609/a-time-out-occurs-when-you-import-large-customization-files-into-microsoft-dynamics-crm

However changing the web.config or registry values broke the CRM. When we tried adding an activity, it threw the plug-in execution failed error written above.

Possible solutions on the other blogs were
1. Running a repair installation of CRM and reconfiguring the service accounts.
2. Removing the plugin solution that's causing this error.

However, In our case, it was the following:
1. Undo the changes to the web.config and delete the registry key added earlier.
2. IISRESET
3. Restart the Sandbox processing service
That fixed our problem, and CRM started functioning normally.