Can you try rebooting the phone?
If it doesn't help, can you capture the issue in the app's debug log (please see my signature below)?
Oh, it's not a Gmail account, is it?
Set up with "OAUTH" (Google's "more secure" authentication method -- used if you choose "Gmail or Google Apps" account type in Aqua)?
I've seen OAUTH fail on mobile data sometimes -- and that's something "inside" Google Play Services, not AquaMail itself... And then K9 doesn't support OAUTH.
If that's the case, you can re-create the account as "Internet mail" (not "Gmail / Google Apps"), and the app won't use OAUTH for authentication.