Support topic: http://www.phpbb.com/community/viewtopic.php?f=46&t=2127933
Someone mentioned elsewhere that custom profile fields are language-specific.
Apparently the code simply assumes that each profile field is defined for the current language, which does not have to be the case if, I'm guessing, a language was added after the profile field was created. Thus it may be the case that there is no profile field definition for the current language.
The code should report this condition in a proper fashion.
It may be worthwhile to also add a note to the page where languages are added, if any custom profile fields are defined, that the custom profile fields need definitions for the new language.