Enabling inactive Outlook add-ins that keep being unloaded

If you cannot enable addin:
1. Remove addin from File->Options->Add-Ins->COM Add-Ins and exit Outlook
2. Run REGEDIT and remove all disabled addins from
[HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Resiliency\DisabledItems]
3.Remove specified addin from
[HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Addins]
4. Reinstall specified addin
For example Microsoft exchange Add-In is located in
C:\Program Files\Microsoft Office 15\root\office15\ADDINS\UmOutlookAddin.dll

Source: www.outlook-apps.com