Error box with exclamation mark

I was in the middle of writing an email using gmail when an error box started popping up every 30 sec or so. It is literally just a black box in the middle of the screen that has an exclamation mark and the word error inside of it, no other explanation to explain the problem. I was able to finish typing my email, but now it’s loading forever when I try to send it. The error box does not show up at all on other apps.