Unsichtbare Zeichen im Eingabefeld verhindern natürlich den Login, das würde sich dann aber in einer Fehlermeldung äußern und nicht im "Aufhängen" der API.

DdlV: give them the op group support address. We had meanwhile reports that several people also experienced a weekend crash of the FXCM API, so something apparently happened with one of their servers. But no one so far had an account that persistently crashes on login.

Is there anything unusual with this account? For instance, single session only, or some particular access restriction?

If nothing helps and the mystery persists, you could continue as follows. Open a new account with FXCM, and transfer all funds from the non-working account to the new account, except for maybe one dollar so that the old account stays open. Then change the password of that account and give us access. We will then debug the login process and can at least provide some info to FXCM what happened, in hope that they can fix it.