Dear Friend:
I also found the problem you mentioned, after several tests I realized that the issue was due to a problem in the messages.inc (maybe that file was corrupted).
In my case I noticed that the problem was every time I select "spanish" language, so what I did was to use messages.inc from another folder and replace on it the right translated variables (in my case) from english to spanish. Then I upload the new messages.inc to the "es" folder and the problem disapear.
I hope this can help you.
Regards
Falconph