cannot load saved project anymore

Hi all,

I've been working with a project in the last days and I have load it several times without any problem. But now I receive this 

ERROR DESCRIPTION:

Error using conn (line 740)
Failed to load file F:\Tesis\NFB_GRX_FM\fMRI\prueba1.mat.
Error in conn (line 4055)
conn('load',filename,true);
Error in conn_menumanager (line 120)
feval(CONN_MM.MENU{n0}.callback{n1}{1},CONN_MM.MENU{n0}.callback{n1}{2:end});
CONN v.18.a
SPM12 + DEM FieldMap MEEGtools conn wfupickatlas
Matlab v.2015b
spm @ C:\Program Files\MATLAB\R2015b\toolbox\spm12\spm12
conn @ C:\Program Files\MATLAB\R2015b\toolbox\spm12\spm12\toolbox\conn

By the way, since I have this problem, I had to change to All Files (*) to be able to see the .mat file of the project when I try to load it. Why is this happening?
I have other projects and there is no problem with them.

Thank you all in advance.

Joanllo