As recently reported by ihop there are times when that message may appear in your browser when posting, clicking on a thread or next page, etc.
Assuming that the "page" is actually available it could result from your browser timing out. Reasons other than a problem with the web site would be an unusual delay on the internet.
However, there are times when the combination of site issues and your particular computer/browser can also cause a time out. It appears that for ihop that may have been the case.
The very occasional 'page unavailable' message will happen. But, as in ihop's situation, it should not happen multiple times every day.
Here is the reply that I sent to ihop:
When I examined the error log yesterday I discovered one particular error message that was logged multiple times a minute that had to do with a Tapatalk file that no longer existed. This file was being checked for even for desktop requests.
I fixed that issue at about 7 PM central time.
I saw your postings between 4 and 5 yesterday in the log with those error messages.
What may have been happening is that as a result of this deluge of error logging, your browser timed-out.
Pending your future posting experience, this problem may have been corrected.