Token information not pulling through to Mailing Template
{membership.start_date} was never supported in token list for sending mailings unless you have any custom extension or module that generates one. 

{membership.start_date} token is supported only in Schedule reminders(for memberships) or printing letters for membership.(i believe). The schedule reminder screen allows you to save the template so that it can be used in future. If you using the same message template to send mailings then the tokens other than contacts won't get replaced unless you have custom code that deals with such tokens. 

Civitokens extension does allow some additional token like latest current membership which can be used in mailings however you will need to replace the existing token with the new ones.

The error message you posted seems like your server doesn't have permission for apache user to download/extract the extension from UI. Alternate way is to download the extension and extract into extension directory and install it manually. 

HTH

Pradeep