Adding a "me too" to this. My Inbox seemed to be fine, but various folders are stuck on "Checking for mail...", which have been fine pre iPadOS 18. I'm running a Dovecot IMAP server on a NAS on my LAN. All other devices (Android phones, PCs/Macs, iPad Mini 1) work fine, as did my iPad Pro on iPadOS 17.
Tried the various suggestions listed so far, none have made any difference. I deleted/re-added an account on the iPad, and now the Inbox (which WAS working) is stuck on the forever "Checking for mail...".
I hope 18.3 fixes this.