You've attempted to upload a plugin in the System Console and it failed with a 400 error and the message
"Received an invalid response from
the server". This can be caused by the max file size configuration setting within Mattermost / NGINX.
This can be related to plugins that are larger than a normal plugin. By default Mattermost limits file uploads to 52.5 MB. By default, NGINX is 1 MB.
You can check this by running
grep MaxFileSize /opt/mattermost/config/config.json. (This file size is returned in bytes.) If this is lower than the size of the plugin you're attempting to upload, then simply increase the value and restart Mattermost.
For NGINX you should be able to do this in the nginx.conf file located at
/etc/nginx/nginx.conf. Add or update the line
client_max_body_size 100m; to be the size you need for the plugin and reload NGINX.
If this does not resolve the problem, you may be able to find more details on why this failed by generating a HAR file and inspecting the Response Content of the 400 error.