OK Ken,
It isn't the most informative message, and it looks to be one generated by your browser, rather than us.
Are you using any special characters in your ad? They can upset the system.
Anyway I have raised a bug report, but I suspect they may not be able to replicate this issue.
You might consider submitting an ad for the mags using the form here instead:
**LINK**
Just scroll to the bottom of the page. I don't need a scan of the form, just email me the essentials, keeping to the word limit.
Neil