Same problem here since yesterday. Changed nothing to the site.
Will try to reset my IE settings, but firefox (pretty never used) has the problems too

Nothing did the trick ...
I downloaded a fresh copy of b2evolution, copied my config file and all went wrong :
http://test.frenchwave.org
I tried to figure ou if this could be a date problem in french locale, "août" is special....
Still problems, even with calendar masked.
Tried to find if this was a referer, a special post, anything... I can't find how to solve.
PS : site link http://blogs.frenchwave.org

Next step : will try with a blank databases....

Found it !!!!

My blog site ( http://blogs.frenchwave.org ) is, on the server, in a subfolder of my main web site ( http://www.frenchwave.org ). Equal for my test site ( http://test.frenchwave.org )

On my main web site, I installed Dragonfly.
DragonFly adds a .htaccess and this .htaccess is working for all sub-dirs.
At the end of this .htaccess is a line :

AddDefaultCharset utf-8

So this charset is also set for blogs ans test.

In the subfolder containing all those site, I added a fresh .htaccess file:

AddDefaultCharset Off

All is nice now.

More insformation : http://httpd.apache.org/docs/2.0/mod/core.html#adddefaultcharset

FrenchW