I just found how to solve my problem :
Here is the answer:
When clicking “Never” in the dialog asking whether to create a portable home directory or not, MCX creates a preference-file (or an entry in the file if it exists already) in /Library on the client machine called com.apple.MCX.plist
This file contains an entry for each mobile user having chosen to never create a portable home directory on the machine. The key is like
MobileAccountNeverAsk-username
in my case it was
Mobile AccountNeverAsk-mobiletest
the value is 1.
After deleting the file, the dialog reappeared at login, and I could successfully create a portable home directory on the machine.
I guess deleting the key/value pair concerning the mobile user without deleting the whole plist file would have done it also.