After starting Microsoft Outlook 2013 or newer the error message "A problem was detected with an add-in and it has been disabled." appears and the MailStore Outlook Add-in is disabled. As a result the add-in is listed in the "Disabled Add-ins" section under "These add-ins decreased performance or caused Outlook to crash."
Microsoft Outlook 2013 and newer are very strict when interpreting loading times of add-ins. Therefore the MailStore Outlook Add-In should be activated permanently. This can be configured in the Outlook options.
In Outlook click "File", then "Manage COM Add-ins" (Slow and Disabled COM Add-ins, see image below), and check whether the MailStore Outlook Add-In is listed. When it is listed, enable it permanently.
The Add-in can be enabled permanently by using Group Policies. The necessary steps are documented in the Microsoft article under "Add-in resiliency / Preventing add-ins from being disabled":
The ProgID of the MailStore Outlook Add-In is MailStoreOutlookAddin3.AddinModule
Article-ID: KB20131212-0-EN (Deutsche Version)