I see you are using Waterfox which is a variation of Mozillas Firefox. I can see that the version number for Waterfox is currently on V46 which other users of Firefox V46 were reporting that there were some major connection problems with. This is something to do with the way that the browser connects to the community and I was seeing the same problems. Since upgrading to V47 the issue has since gone away (we are still keeping our eye on it though). I think V47 is on its way soon but I can't find anything about when that will be.
Just to be clear, there is no issue with having a # in the subject line or content, the OP was quick to jump to this conclusion when presented with an error. The real problem lies with the version of Firefox that is being used.
I would hang tight until V47 is released of Waterfox or try using a different browser in the meantime. It might be that the core of Waterfox can be updated through the automatic updates but I don't use the browser so I can't tell you for sure if that's the case.
