It's probably because the method of creating image assets in Edit+ actually happens in 2 steps. The first step is to create a blank image asset in Matrix backend, Edit+ then uploads the actual image and puts it into that created image asset. So your trigger is probably firing as soon as the first step completes, and interfering with the 2nd step Edit+ needs to make, and thus failing the upload.
Can you use a different trigger such as set future status? So that it goes live in the next cron run perhaps? Not sure if that trigger will still interfere.
Another way would be to use trigger method for creating image varieties in Edit+, but instead of using the image variety as the action, you use the set status action. Here are the steps for configuring the Event and Conditions for it:
- In the Events section, select Simple Edit Accessed.
- In the Conditions section, select URL matches in the list provided and click Commit. Additional fields will appear in the Conditions section. Enter a URL pattern match of file_create=true and select Must be in URL then click Commit.