Invalid file type, blocked file type, can’t save settings

Hi,
I’m suddenly not able to create new document entries – says ‘file type blocked for security reasons’. Also, I can’t download previously uploaded documents – says ‘invalid file type’. In settings, I tried to add file types in case this was a new requirement -but it won’t save when I change the Basic tab (the other settings pages save ok). Any help much appreciated – the plugin basically isn’t working for me right now.