Error with addressing a subtask

[Jesse Feliciani]

I've created some custom subtasks for our issues. I can select the subtask from the dropdown I want to use after clicking on 'Create subtask'. But once I click on the subtask in the parent issue, there's a yellow warning triangle with an exclamation mark on it, then when I try to open it to address the fields I get an error stating, " We couldn't create the subtask. Check your connection, then give it another try."

I've closed the browser, closed all tabs, restarted...but the same problem keeps popping up.  

Anyone else come across this or have a fix?