When should this be used?
When a problem goes away and nobody knows enough about why that an answer would likely help anybody else, voting to close as off-topic with the "a problem that can't be reproduced" sub-reason is appropriate.
Often, when the OP solves their own problem, people vote to close as "a problem that can't be reproduced." That's wrong. When the solution might help others with the same problem, the question should be answered.
This is actually totally off: firefox can't the server at start.unbuntu.com
It was asked 18 hours ago. He replied 7 hours ago. He has not said that he can now access
start.ubuntu.com. I can't see anything there suggesting that his issue has been resolved.
This is another example:
Okay, so it was asked a while ago. But how has it been resolved? Has anyone asked the OP if they have solved the issue?
Reminder. The only time you should use this reason is when a problem goes away, and the solution is not known.