Hello,
this issue means that your server limits the execution time for scripts and sitemap generator cannot complete crawling, the workarounds are:
- increase max_execution_time setting in php.ini file at your server and restart apache
- execute sitemap generator from command line (if you have ssh access to your server)
- use "Save state" option at Sitemap Generator configuration page and execute the crawler multiple times with "Resume generation" enabled until full sitemap is created
- limit the number of pages to index in Sitemap Generator configuration