I Already resolved the problem
It seems like the mbox file for the user was corrupted. He could read all the emails from the web and could send and receive emails without attachments, but he couldn't receive emails with attachments. What I did was the following:
1. Make a backup of the mbox file.
2. Remove the original mbox file and create an empty new one.
3. Create a new account called whatever I want, and replace its mbox file with the one I backed up before.
4. I made a fetchmail of all the emails using pop3 protocol from the new created account to the account with problems.
This way the new mbox file contains all the emails and is not corrupted. After all this everything worked fine.
Thanks for your attention and best regards.