Whatever this is, it seems to be a local problem (ie specific to you or your network). I was just able to post to the main Plugins forum without any problem. Other than deleting/deactivating any browser add-ons, I can’t think of anything to suggest other than to wait 24 hours and see it it resolves by itself.
The good news is that there isn’t much point posting to the main Plugin’s forum in my opinion. Your topic is unlikely to get picked up. Ideally, you should be posting to a sub-forum that’s dedicated to the plugin you are having problems with. You can locate any plugin’s dedicated sub-forum via its page in the Plugin Repository.
If it’s a general plugin question (like “How to I upgrade a plugin manually”) then the Troubleshooting forum might be a better option for you.