Having this error also, trying to upgrade the Covert Store Content Plugin To Version 1.6. It says it is compatible with your upgrade tool, so maybe it is something with my server?
https://covertcontent.com/store/plugin_update/std/update.html
“Unable to initialize WP_Filesystem. Will not proceed with the upgrade.”
“The Easy Theme and Plugin Upgrades plugin was unable to handle requests for this upgrade. Unfortunately, this setup may be incompatible with the plugin.”
Trying to use FTPS (SSL) to upgrade with password. Can you please follow me @fratdeals on twitter?