Hi guys,
If that same issue occurs again, please capture it in a debug log and send it at support@aqua-mail.com. If there are any error messages, please provide a screenshot of them too.
For the debugging log:
1. Open Aqua Mail > Settings > Debug
2. Enable "Debug logging"
3. Enable "Log raw data"
4. Enable "log database data"
5. Reset the log file if you've used it for something else before
Go back to Aqua Mail > perform the exact same steps leading up to the issue. You should disable logging after that by going back to Settings > Debug. The file is located in your phone's internal storage - the default directory.