I too have been unable to use Bullguard BackUp - but not online - I've been trying to backup to an external hard drive and get the same messaghe that you are getting.
I have been in contact with the support team who have done a remote access on my PC and investigated some BackUp logs from my PC - and I am waiting for a response from the techie guys investigation - their initial thought is that it may be a problem with one of the files i am trying to BackUp.
By the way , must give the people at Bullguard a pat on the back for the support and perseverence in trying to solve the problem.