The initial setup of the first email account uses a Account Wizard (AKA Account Assistant) which attempts to guess everything based on only an email address and password and type of account (imap/pop).
However, any error in this process flashes the error message too fast to read, and returns to the input window with no error message and the account is not set up.  User is forced to set up account manually.
The error coud be in the entered data, or it could be in contacting the server.  In my case I was trying to set up an yahoo email as an imap account.  All input was correct, but wizard could not make it work and the error flashed so fast it could not be read.

Note: English menues list this wizard under Tools / Account Wizard.  However when it launches the window it is titled Account Assistant.

Reproducible: Always

Steps to Reproduce:
1. Invoke the Account wizard for account setup on a common mail provider
2. induce an error on the account assistant screen
3. Click next to select account type, etc
4. Error message flashes extremely briefly - too fast to read

Actual Results:  
Regardless of the source of the error, the error message flashes too briefly to read.

It was necessary to set up the account manually, which worked fine.

Expected Results:  
Stop with error message showing so that user can take the propper corrective action

Kmail 5.1
KDE Frameworks 5.17.0
Qt 5.5.1 (built against 5.5.1)
The xcb windowing system
Manjaro (archlinux) repositories
VmWare Virtual machine