AH-HA! I figured it out! It's a mix of mobile view and the letter "u" being used by itself. Our system doesn't allow people to type the letter "U" by itself, as people use it as slang for "you," which can confuse people who speak english as a second language.
In the desktop view, it tells you:
We're sorry, but your post appears to contain abbreviations that we don't like people to use at the Ranch. Because JavaRanch is an international forum, many of our members are not native English speakers. For that reason, it's important that we all try to write clear, standard English, and avoid abbreviations and SMS shortcuts. See here for more of an explanation. Thanks for understanding.
If the abbreviation occurs within code, you can use code tags to post it successfully. If the abbreviation is a variable name, you can use the tt tag.
The specific error message is: "u" is a silly English abbreviation; use "you" instead.
BUT, in the mobile view, it doesn't tell you ANYTHING. It just doesn't submit. It sits in limbo and you have no idea why.
Mobile view needs work.