This error has two potential causes.

Either the Online Reader in your browser failed to send the authentication information needed back to our server, so we can be sure that the person opening the ebook actually has that ebook in their account.

Or this particular request (the URL you are using to attempt to open the Online Reader) has already been used elsewhere. Either you have tried to open the book on two different devices, but by using the same link. Or you saved the URL of the Online Reader as a favourite in your browser and are trying to use the favourite to open the ebook (please do not do this, please sign in to eBooks.com and access the ebooks from your bookshelf).

Regardless of the cause the solution should be the same. Simply close the Online Reader and reopen it.

If the "Session Ended" box pops up, simply select "Reload and resume reading".

If this still doesn't work please take a screenshot, including the full URL displayed in the browser's address bar and please complete our support form to submit a ticket to our customer support. Please be sure to mention the error message (403_FP_MISMATCH), and the title of the ebook that is having the issue, and attach the screenshot. Please also copy the URL from your web browser and paste that in your email. Please also mention that you have tried the solutions suggested in this help article. We can then investigate and resolve the issue.