From what you describe, it is possible that your version of Mame is so old that it doesn't match the catver.ini, history.xml AND history.dat (if you can find it), nplayers.ini...files that are used (and automatically downloaded) by mGalaxy to build the database.
First, I suggest you to delete these 3 files from the MAME application folder (where mGalaxy places them)
Then try to rebuild the database!...and tell me what happens!