Wrong UMCP parameters, wrong locale?

On my UCS 4.2 systems I get the following error in the UCM:

Ein Fehler ist aufgetreten:
Webfrontend-Fehler: Die angegebenen UMCP-Befehl-Argumente der Anfrage sind nicht zulässig.

Fehlernachricht des Servers:

Specified locale is not available

First I thought my test system might been somehow broken. But actually I get the same error on my productive system.
I can not remember, what I have changed. I ran an update a while ago… But did encounter this problem only today!

Thanks a lot for any hint.

OK, it looks like a nice Chrome Plugin “quick language switcher” actually sends language strings like “en.UTF-8”, that UCM does not like.
I was able to change the languages in the language switcher to “en_US.UTF8” and UCM stopped complaining!