Hello there
if you try to change the sql server you have to re enter the database root user.
The user is disabled by the imscp setup (since 1.3.x)
If you want to change the sql server (e.g. 5.5 to 5.7) you have to reenter the root user.
The root user cannot login and the update / reconfigure fails.
greets
/edit
sql manager -> sql server