I have a solution.
For Sitemap Content,
– Uncheck “Include tag pages” box if it is checked.
For Basic Option,
– Increase your memory limit. (eg. 8M, 16M, 32M, 64M, 128M, 256M, 512M)
– Limit the number of posts in the sitemap (eg. 100, 500, 1000, 5000, 10000)
Play with the option. It will work.