Can't add page, get Forbidden on form submit

HOSTING SETTINGS

PHP: 5.4.40
Zend: 2.4.0
OS: Linux
SAPI: cgi-fcgi
Safe mode: not detected
MySQL client: mysqlnd 5.0.10 - 20111026 - $Id: c85105d7c6f7d70d609bb4c000257868a40840ab $
MySQL server: 5.1.73-cll
Extensions: Core, date, ereg, libxml, openssl, pcre, sqlite3, zlib, bcmath, calendar, ctype, curl, dom, hash, filter, ftp, gd, SPL, iconv, session, json, mbstring, mcrypt, standard, mysql, mysqli, mysqlnd, Phar, posix, pspell, Reflection, imap, SimpleXML, soap, sockets, exif, tokenizer, xml, xmlreader, xmlrpc, xmlwriter, xsl, zip, cgi-fcgi, PDO, pdo_sqlite, timezonedb, pdo_mysql, ionCube Loader, Zend Guard Loader
GD: Yes
ImageMagick: No
PHP max upload size: 30M
PHP max form post size: 30M
PHP memory limit: 180M
Total max uploadable file size: 30M
Resource folder writeable: Yes
Session timeout: 24 minutes
Native JSON: Yes
Filter functions: Yes
Transliteration functions: No

It's a hosting server, and it's a garden variety shared hosting plan. I've put quite a few Perch sites on the same host, and the site in question has been in service with Perch for over a year. Something must have changed on the server, but I'm at a loss as to what to investigate or try next. It's especially vexing that the "add page" Perch form would load fine with no data, but then show a 403 Forbidden error on save. Isn't is the exact same file that's reloading on Submit?

It's the standard Perch "add a page" form in the control panel. /admin/core/apps/content/page/add/

Trying to add a "link only" page with title "Ultimate Plant-Based Nutrition Resource Guide" and url "https://resources.plantricianproject.org"

Okay, thanks, I wouldn't have thought of that. It does seem a little spammy :-) I'll ask the host.