Hi,
It might be due to some corruptions. Indeed it is not adviced to copy paste entire rows: even if you just have a few columns displayed in your table, it will copy all task information such as enterprise custom fields.
Do you have the same issue on a blank new project?
You might try to repair the project with the save as sharing or save as XML methods.
It might be possible that the Enterprise global table that was assigned to most of the views is probably corrupted.
Try to open the enterprise global and made a copy of this table with the same fields and attributes, then replace the old table with the new one in all the relevant views, and see if it works!
Hope this helps,
Guillaume Rouyre, MBA, MVP, P-Seller