Mail: Mail was unable to open this mailbox ... / Partition pending requests queue is full

I recently updated to 13.1 and am now receiving these error popups from Mail on my MacBook Pro:

The message “Sam's Club Membership Packages and More” could not be moved to the mailbox “Mike/iCloud”

Mail was unable to open this mailbox on the server “p53-imap.mail.me.com”.

The server returned the error: Partition pending requests queue is full: partitionMaxPending=128 (took 19802 ms)

I have 1.6TB of iCloud storage available. My iPhone continually spins while trying to check my iCloud email (but doesn't show any errors).