This is a cookie/caching issue, where the ebooks have been removed, or the list has been updated, but your browser has stored the webpage information and so is not showing you the latest version. When you click the remove button the website then shows an error because as far as it is concerned the ebook was already removed.
If you hold Shift and then click refresh on your browser that should resolve the issue. Otherwise please try the following:
In order to clear your cache, please follow the instructions for your browser, which can be found here.
In order to delete your cookies, please follow the instructions for your browser, which can be found here.
Once you have done both of the above visit the page again and it will have updated. If not please complete our support form to submit a ticket to our customer support. If you are trying to remove or add ebooks please state which ebooks.