Additional Info: After Server reboot it's the same again. I noticed, that the localdomain account - the domain of the logged in user - works. The IMAP's server name here is 127.0.0.1 which is, I suppose, the default. Adding an IMAP account from a remote server works too.
The local server is serving a couple of domains. Adding an account from one of the other local domains, I can't set the server's name to localhost nor to 127.0.0.1. When I use the IP address of the local server or the MX's DNS name, these accounts don't work as described.

I added the hosts IP with mx name in /etc/hosts and now it seems to work.

I had the same issue for some time now with latest sogo 5.8.0 installed using iRedMail 1.6.2. The last comment by nuwohg got me thinking and instead of using 127.0.0.1 as used by the default imap account, I used the FQDN of my mail server. This seems to completely resolve my issue. However, one time I was able to see the IMAP folders of a second Account added using localhost and was even able to send mails from that imap account, but I am unable to reproduce is. I suspect that this is some weird interaction with the mail server and sogo.

TL;DR
Use the public facing FQDN of your mail server and it should work flawlessly.