Need more information before anyone can provide any help.
- What Roundcube version are you using?
- Have you tried disabling all plugins?
- What is in your RoundCube error log?
- What are in the emails that you can't open?
2. have no plugins, $rcmail_config['plugins'] = array(); - empty here
3. PHP Fatal error: Class 'DOMDocument' not found in /usr/local/roundcubemail-0.9.4/program/lib/Roundcube/rcube_washtml.php on line 364
4. I can't open different emails, they have no same "structure" and same provider - just absolutely different - some of them ok some 404