While trying to update the plug-in to the latest version (3.5.9) from WP site admin dashboard, it returns the following error: An error occurred while updating Microthemer: The package could not be installed. PCLZIP_ERR_BAD_FORMAT (-10) : Invalid archive structure.
I updated a few sites this morning and all went okay so I think it might be the update is being interrupted. I’d suggest trying again after a little bit as WordPress update servers could be having issue, or download and upgrade manually.
Sorry for the trouble guys, I’ve re-uploaded the zip package and the auto-update works on my tests now. I think the zip file only partially uploaded yesterday which would explain the PCLZIP error message.