this is most likely related to the maximum script execution time limitation at your server.
You should either:
1. (preferred) modify your php configuration by increasing the max_execution_time setting in php.ini file and restart apache
2. execute sitemap generator from the command line (if you have SSH access to your server)
3. Use "Save state" otpion in Sitemap Generator configuration and after the "page cannot be displayed" message Restart the crawler *with "Resume session" checkbox enabled* you may need to resume it multiple times until full sitemap is generated.