Ich geh mal davon aus das Du in der richtigen Reihenfolge neu installiert hast, bei deiner Version musst Du erst die

1.) GIANTS Ultimate "Data Files".exe installieren und danach die

2.) GIANTS Morrowind.exe

Falls ja, könnten die Fehlermeldungen damit zusammenhängen das Du vor der Neuinstallation dein Savegame nicht gecleant hast:
Zitat Zitat von GIANTS-readme
Clean your save game of ANY previous version of GIANTS.
This is done by going into a interior cell (preferably a small shack with no creatures in it at all) and saving your game. Exit MW. Before you start your game again UNcheck GIANTS.esp from data files. Then load the save you just made. There will be many error messages so just ignore them and keep clicking yes to continue. Once done clicking save the game again. Exit MW. Install GIANTS Ultimate and check GIANTS esm in data files. Now load up the save and everything should be okay.

Note: To prevent headache of clicking "yes" a thousand times I suggest you change your Morrowind.INI file with this little tweak. In the Morrowind/Data files folder find your Morrowind.ini file (always make a back-up when editing) and add these lines:
In the [General] section:

AllowYesToAll=1

This will add a "Yes To All" button to the warning popups.