A technical problem with the server created an error. Try again to continue what you were doing.

[alexander_rios]

Same Issue in Magento 2.3. I found the correct file in vendor/magento/module-email and edited line 16, replacing "fileUploader" with "imageUploader" and that fix the issue.

[lee_derting]

Is useful for me :

https://tutes.in/solved-magento-2-error-a-technical-problem-with-the-server-created-an-error-try-aga...

[jgwenzel]

So, I had this error because of a bad filename that had an equals sign in it. I didn't realize it because my eye's are not great. Hope this helps someone.