Why is the Outlook Plugin is not loading in Outlook Addins menu even after installation?

This may be because of some registry editor problem in Windows machine while installation. In this case, we can follow this process to clear Outlook plugin from the machine

Open Register editor:
1) exit from outlook
2) uninstall the plugin from Control Panel
3) Press (Windows + r) to open run prompt
4) In the Open box, type "regedit"

Clearing Outlook Plugin registers:
5) If Outlook is 64-bit then, expand HKEY_LOCAL_MACHINE/Software/Microsoft/Office/Outlook/Addins/
    If Outlook is 32-bit then, expand HKEY_LOCAL_MACHINE/Software/wow6432Node/Microsoft/OfficeOutlook/Addins/
6) delete VtigerCRMOutlookAddin.connect if you find any thing there

Clearing Outlook Plugin disabled plugins cache:
7) locate HKEY_CURRENT_USER\Software\Microsoft\Office\version_number\Outlook\Resiliency
8) remove every thing from Resiliency register
9) Now we cleared every thing related to our plugin from the machine. Open Outlook, we shouldn't find any error from Plugin
10) Reinstall plugin again

To know more about the Microsoft Outlook plugin, click here.