Can't open anymore shp-sqlite

Hi,

Since version 5.1.1, my plugin is not working anymore : I can't open shp or spatialite with a code which was working before. I get messages as
'shp.Callback' threw an exception of type 'System.IO.FileNotFoundException' when files are really there and can be opened with Qgis (doing a cut and paste with the full path copied from the code)

for shp I use
FeatureSet shp = new FeatureSet(fileName); /

for spatialite
var layer = new VectorLayer();
layer.Open(fileName,table);

What could be the problem ?

Thanks,

Olivier