Christian, while i agree with you on the update frequency, an audit would at least prove that you guys are capable of designing a piece of software that worked well and securely *in the past*.
This - at least - would reassure, that you would be able to develop a good piece of encryption software in the future.
Without an audit, no one will know if your code and your security setup a) really matches the technical overwiev (as bc is closed source) and b) has been secure enough in the past.
An audit would furthermore be a sign of confidence in your own software.
So far, Boxcryptor has not been audited, is not open source, has unresolved flaws (just look at this forum!) and is quite expensive in the premium version...i strongly suggest spending the money you earn from us to get a strong security audit.
Without an audit or a FOSS client, i don't trust your software enough but to use it for some unimportant files...which prevented be from renewing my premium subscription.
Today, you are only able to do successfull business because the competitors are either dead (cloudfogger), possibly flawed by NSA (viivo etc...) or not mature enough (Cryptomator).
Once Cryptomator is mature enough, you are running into serious problems. At least my own company will switch immediately to a FOSS product...