Reinstall gdm and gdm themes.
Or, open lxdecc (Configure your LX Desktop) > System tab > File Manager Superuser Mode. Open /usr/share/gdm/themes/Avio-GDM/. Right click Avio-GDM.xml and choose to open with leafpad. Check that line 4 has
Document : AvioGDM.xml
If there is anything but AvioGDM in that line, change it to AvioGDM.
Following the same procedure, open GDMGreeterTheme.desktop. Lines 2 and 3 should read -
If it has something else there, change it.
To see line numbers in leafpad, click Options > Line numbers.