Notices

I have always had a problem that Modest will stop updating my IMAP Inbox. The email account will show the last updated date as an unchanging date or time. Asking it to send and receive does not change the updated date and the messages in the Inbox do not change. However other IMAP folders on the same account continue to update normally.

In the past I have deleted the account when this happened and then recreated it. Doing so allowed it to work for a few days. Recently I started deleted the contents of ~/.modest/cache/mail/imap/* when Modest failed to update the Inbox. After the deletion Modest will update the Inbox for a few days.

The Inbox contains around 5500 messages.

One of the 60 sub-folders of my IMAP inbox currently has over 9,400 messages. I've not noticed any problems so far. Due to server-side filtering I've never had more than about 50 messages in the root folder itself which, as you suggest, may be significant.
FWIW, I used MohammadAG's patched Modest on PR1.2. On PR1.3 the default Modest install is fast and reliable enough for my needs.