Hullo all,
Thanks for all your patience. We realise that this must be vair annoying. Tech haven't been able to replicate the problem yet, so as an interim solution, we'd suggest using an alternative browser, such as Chrome or Firefox.
We have a special mission from Tech, should you chose to accept it, that could help us figure out what's happening in IE.
When you open IE, in the bar below the address bar, there's a Tools tab. Click this, and then go down to F12 developer tools (or you could hit F12, as I've just figured out)
This will bring up a box at the bottom of your screen. Choose the "Console" tab. Then, when you next try to post on Mumsnet and it's not letting you, could you take a screen shot of what's happen in the console box at the bottom of the screen, and send it to [email protected]? The have a 
Have attached some pics to hopefully make this a bit clearer. Do keep us updated on what's happening, and we'll try to get this sorted asap 