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(this part is non-translatable btw...) and then shows the error, which in this case is just:
using uselang=debug, I can see that this is a translatable string, .error.loginfirst but I don't see that string (nor /interests.bml.error.loginfirst) in the translation interface. A do see a /interests.bml.error.enmasse.mustlogin string however (and also /community/join.bml.label.loginfirst but that is probably irrelevant).
On top of that I just see that the spellchecker corrects the 'occured' in the "Error ..." part to occurred?