[FieldTrip] connectivity analysis with rereferenced EEG data

[Victoria Schröder]

Hello

I am trying to do a connectivity analysis with Fieldtrip. I recorded 
the EEG data with a BioSemi system without choosing a reference channel.
Thus i need to select a reference in Fieldtrip. I did that during 
ft_preprocessing(cfg) by using the following code:
cfg.reref='yes';
cfg.refchannel='all';
Data=ft_preprocessing(cfg);

however when i later want to do the ft_mvaranalysis(cfg, Data) i get 
the following error:
Matrix must be positive definite

I read that this error probably occurs because the cfg.reref procedure 
changes the ranks of the data matrix.However, i need to rereference my 
data.

Do somebody know a solution?

All the best and thank you in advance
Victoria