TL;DR: Cannot reproduce with a 400 page long plain text without markdown. Would you mind sharing more info on the content of the page?
Test:
Created a new blog page using Lorem Ipsum generator: Generated 1500 paragraphs, 132950 words, 898900 bytes. That takes roughly 400 page scrolls on a 1366 x 768 screen…
Site: Skeleton blog site.
Results:
Browsing to blog home page:
Server Error
Sorry, something went terribly wrong!
0 - Maximum function nesting level of ‘256’ reached, aborting!
For further details please review your logs/ folder, or enable displaying of errors in your system configuration.
Log page grav.log:
grav.CRITICAL: Maximum function nesting level of ‘256’ reached, aborting!
… Grav/Common/Helpers/Truncator.php(173)
It fails to create a proper truncation to display as summary.
Custom summary:
Only options I know are page.summary() and page.content(). You might try to add a custom summary option into frontmatter and adjust blog-list-item.html.twig (Quark), like: