Hi Ian,
Can you please create a debug log which contains information what happens when the problem occurs? Here are guides how to create a debug log:
https://support.boxcryptor.com/display/BCWINEN/How+to+create+a+Debug+log
https://support.boxcryptor.com/display/BCOSXEN/How+to+create+a+Debug+log
Just send this debug log to support@boxcryptor.com referring to this forum post and we will track down your issue.
Best regards,
Robert