[Qgis-user] Attribute table not visible only main QGIS window visible on Ubunut 14.04.1

[Pascal Dupont]

On a first computer with QGIS 2.4 and 2.2 on Ubuntu 14.04.1, the attribute table window is not displayed, only the main QGIS window is visible.
I mark the layer and press the icon attribute table. A small window "Attribute table. x features loaded. Abort" shows the progress in loading all features of the big layer with some 1 million features. After a view seconds this window dissappears.
When I press the keys ALT and TAB, a QGIS icon is shown. When I mark the QGIS icon and wait a few seconds a preview of the main QGIS window and a preview of the attribute table window is shown. When I select the attribute table preview window by pressing the tab key while holding the ALT key and release the all keys then the attribute table window is not shown. If I reduce the size of the main Qgis window or move the main Qgis window on the screen, I cannot see the attribute table. When I close the main Qgis window, I cannot see a Qgis icon by pressing the keys ALT and TAB.

On a second computer with the same software, a warning 
   Update of view in private qgis.db failed.
   database is locked
appears, but then the main qgis window and the attribute table window are visible.

How can I view the attribute table on the first computer?