WebmasterVPU

Solved.

Thanks for the answer.
I have also connected with the plugin owner and they are working on a fix.
And yes, I am aware that modifying functions.php is not a sustainable solution. Though, does the trick until the plugin is fixed.

Thanks for the quick fix.
Great support!

Unfortunately, we have already the max allowed by our provider. The strange thing is that it used to work fine and – as I mentioned before – there is no issue with posts (where there are much more), only with pages. Even CPT work.
Guess I’ll deactivate the sitemap feature.
Thanks.

Unfortunately the plugin still does not work on post titles which was the reason to install it on my site.

Yes, our default wordpress installation has already SI Captcha Anti-Spam included.