When I have seen first time this error on Joomla 1.6 while tried to save a new article in Article Manager I was lost:
Save Failed with the Following Error: Invalid Parent ID
I was diggin' around to see what can be wrong, and the final clue
leading me to the solution where come from a phpMyAdmin checkup. Weird
thing was that the article has been saved, but I found an unexpected
value in the article's database record: the article wasn't assigned to
any category ID and found a value which I wasn't knowing back then what a
heck is for: a column called asset_id with value set to zero.
And this clue has been leading me to the solution (BTW, first step
with rebuilding the menu might be unnecessary, but I decided to leave it
there - because can't harm, but might fix similar errors in menu system
you are not aware of - yet):
Go to the Menu Manager – Click on Menus, Menu Manager, Click on the ‘Rebuild’ button
You will see a message that says:
Successfully rebuilt
Go to the Category Manager – Click on Content, Category Manager, Click on the ‘Rebuild’ button.
You will see a message that says:
Categories tree data successfully rebuilt.
Edit all your parent categories and save them.
Edit all your sub-categories and save them.
Go back to the Article Manager, edit all the articles that were
created with the ‘Invalid Parent ID” error message and save them again.
After refreshing your menus, categories you should not have any
problems creating new articles anymore. As to the existing articles that
had asset_id = 0, those asset_id’s will get populated after you open
and save the articles.
As you might guessed allready, the error was generated by the fact
that the category tree structure was messed up for some reason.
