page update fails after high number of elements

Dear Kriesis,
I am creating a page with a large number of elements. Once a certain number is reached, I can no longer save the page. I am redirected to “Page not found.” Other pages with fewer elements are not affected. The website with this particular page is part of a WordPress multisite.
The server settings are:
max_input_vars = 10000
upload_max_filesize = 100M
memory_limit=1000M
max_execution_time=360
max_input_time=360
post_max_size=100M
Could it be due to the theme? Are there any settings I can adjust?
Best regards Anne