QGIS web client installation: this.projectSettings.capability is undefined

I am trying to debug my configuration of QGIS web client.

I have the same problem illustrated here (the GIS-Browser appears but it keeps loading map for the two sample projects ). Yet following the instructions in the same post doesn't solve the problem for me.

The apache log doesn't indicate any error, just warnings

Warning 1: Unable to find driver ECW to unload from GDAL_SKIP environment variable.
Warning 1: Unable to find driver ECW to unload from GDAL_SKIP environment variable.
Warning 1: Unable to find driver JP2ECW to unload from GDAL_SKIP environment variable.
Warning 1: Unable to find driver ECW to unload from GDAL_SKIP environment variable.
Warning 1: Unable to find driver JP2ECW to unload from GDAL_SKIP environment variable.

Yet debugging the page where the maps don't load I get the JS error

TypeError: this.projectSettings.capability is undefined

I can't find any explanation of that.