Illegal choice and internal server errors when adding a category

This might actually be an error with the way I use the module, but here is how to reproduce this error:

1.) Create a container
2.) Add several categories (with weights in descending order) by starting from the 'add child category' link on the container page.
3.) Go to one of the category pages
4.) Click on the add child category link
5.) In the parent category box, choose the category to which this category is being added as a child
6.) Submit the category

The following error message is returned:
An illegal choice has been detected. Please contact the site administrator.

If I persist and choose the parent category again and submit, I either get an internal server error or the same error message. The role adding this stuff is uid1, so should not be a permissions issue.

This error does not occur if I add the child category from create content>>category, select the right parent container and then the right parent category and save the node.

Speaking of internal server errors, I am also attaching a screenshot that is unrelated to the current occasion but might be the same bug (I don't remember).