I guess you use Runbox 5 Enhanched. If so, then you will find the setting for auto-complete on nicknames in Preferences. Check if it is on?
https://beta.runbox.com/mail/options
Regarding the other problem, do you still get that? I dont see any reason why you should get it based on the amount of emails you have in each folder. Could you try with RMM5 Basic and see if you can spot the problem there too? You can change versions down to your left. If you dont want to test RMM5 Basic, then if you get the error, please write down the time and what you tried to do. Then send us a Ticket as fast as possible and we will investigate what lead to a timeout.
Kim