I just found out about the reason a problem I faced for several weeks: 
When managing my personal sieve scripts using kmail2s interface I always got a "Error unknown" whatever I tried. Managing the scripts had worked before, actually I setup the scripts using kmail2s interface. I failed to find out what the actual problem is. 
Today I finally used 'sieveshell' to give it another try and was presented a clear and easy to understand error, when trying to upload a modified script: "you are only allowed to setup 5 scripts."

Wow. Ok, that is something that actually helps!
1.) I know what the problem is
2.) is was a question of seconds to fix the problem
3.) I was able to manage my scripts again
So my question is: why does kmail2 not display that error message? Why the "Error unknown"?

Reproducible: Always

Steps to Reproduce:
1. Use a cyrus imap account and maintain sieve scripts, more than are actually allowed on the server
2. Try to make _any_ modifications to the existing sieve scripts using kmail2s script manager
3. Shake your head at the great error message you get
Actual Results:  
I get a meaningless error message: "Error unknown" for whatever modification I try, except activating a script or another. 

Expected Results:  
I would expect to get the error message the server generates. This is what error messages are there for. It actually sometimes helps users to give them the error message.