I have tried to do this on, wait for it ..... a Windows laptop (Chrome), an iMac desktop, an iPhone, an iPad (the one I won on GN ) and on my 5 year old iPad. All of the Apple devices use Safari.
When I click 'save' the page refreshes, but the 1000 has changed back to 100.
Also, I gather from comments on another thread that there should be an option on the green bar at the top of the thread which reads "Show all messages". I don't have this. If I go to Customise and select 25 messages and save (it does work for this!), then I have an option on the green bar which says "Show 100 messages".
I've tried turning Adblock off, but this makes no difference.
Wait, I'm going to sign in on IE and see if that makes any difference.....