Hello,
I have seen this before when there is a custom field configured to be required in server settings. If the field has no value then any other updates to other project fields will fail in Bulk Edit.
Check your Enterprise Custom Field configuration in Server Settings and if any fields are marked as required make sure to include that field in the Bulk Edit view set that field value first for all projects. Alternately disable the required field – personally I use workflow controlled always in place of required due to this and other issues caused by required fields.
HTH,
Martin