One day I tried to log in to my Joomla 1.7.1 frontend area as a member of the site. I entered the correct contact information, but on clicking "Log In" I just got a white screen with the message "Fehlerhafter Code". Google only wanted me to update the page (which needs to be done sometime, I know...) - so that was not very helpful.
Suddenly I remembered that I had changed the language settings of my page some time ago - it was German before, and I changed it to English. So far, so good, but I apparently forgot to set the default language for the frontend - I removed the German language pack so that only English was left. But I did not click on the "default" star icon (needs to be yellow).
Go to "Extensions" - "Language Manager" - select the tab "Installed - Site" and make sure you have a default language selected.
When I returned back to the frontend, I cleared the cache and everything worked fine again...