Suddenly stopped working on one site and caused fatal error

Hi, I have been using your plugin on a few sites and it is excellent however I just noticed it had stopped working on one site (the sitemap page was blank except for the site header area, but the footer wasn’t showing plus no sitemap showing either). I tried to open the sitemap page but trying to open it has caused a fatal error, so then I tried disabling your plugin and re-enabling it – but then I couldn’t re-enable it, as it then caused another fatal error, and the message was:

Plugin could not be activated because it triggered a fatal error.

Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 20480 bytes) in /home/sites/15a/b/b13d8a8d50/public_html/wp-includes/class-wpdb.php on line 2879

I don’t have time to troubleshoot it, and so I’ve had to install another sitemap plugin, but I just thought I’d notify you of the issue (…which could be due to the recent plugin update, as it was working fine before, and it is also still working ok on my other sites too) as I noticed someone else also suddenly had a similar problem so I thought I’d let you know – Thanks