Unexpected error while saving label .... Application_fi.properties.xml

[Veli-Matti Hurskainen]

When trying to save a value to the language file I get "Unexpected error 
while saving label .... Application_fi.properties.xml".
This did work just fine until today when the error suddenly appeared.
The file itself seems ok, it's (rw-r--r--) and owned by "nobody".
I've stopped and restarted the server, no change.
Can anyone give a hint, what's going on?
VmH