Must be a Gmail account?
Gmail has a limit on how many connections it allows at the same time -- counting all mail apps (except Google's own) on all devices and all platforms (mobile, desktop, etc.)
And my app is very honest with reporting errors.
Usually, though, you might run into it with too many folders in push mode (each requires a connection) and/or network disconnects (e.g. elevators, underground parking lot at the office, or just poor mobile data coverage).
Scheduled mail checks use only one server connection per account, although that can disconnect too, subject to same causes.
Can you check that you're not using push mail for this account? Sounds like you prefer scheduled mail sync.
To check: long press the account in Aqua -> options and folders -> account options.