Joomla 2.5.27-31: Can't edit article any more

My Joomla installation just got updated (automatically) from 2.5.6-3 to 2.5.27-31 and all the formatting of the frontpage got lost. Trying to re-format it, I discovered (using TinyMCE editor), that I can format it, but once I hit "Save", the whole text's formatting gets reset to the previous state.

I'm using the beez5 theme, and aside from some modules and plug-ins (i.e. Facebook and Twitter integration), the installation is pretty much stock.

Any help would be highly appreciated.