Hi there @aschx, Just reading parts of this thread to get caught up on the issue.
What version of WordPress? Multisite ore regular?
What other plugins are you running? Specifically, what other plugins might be also trying to filter media items?
Are you seeing the error both when uploading new images as well as bulk smash.it actions on existing images?
Try this. Go to Settings > Media on that page you will see a section for Smush.it. There is a checkbox to enable debug mode. This will output some details on the plugin API communication with Yahoo. Save the settings.
Next go to Media > Library. Find a PNG item in the listing which has not be processed via Smush.it. Set the checkbox in the first column of that row. Above the listing is the actions drop down. From the drop down select Bulk Smush.it then click the 'Apply' button next to it. This will bulk smush a single image.
You will see some debug output like the following
Also, I enabled auo-smush.it on upload and get the same success results.
More then likely this is something with your specific server or WordPress configuration. As we are not seeing the same errors when attempting to process the same image.