Import second calendar for a user in kolab 3.4

[Pietro Vassalli]

I have a brand new installation of kolab 3.4 on ubuntu 14.04.04.
 A user is importing his 4 old calendars.
 The first is imported without issues.
 Then he creates a new one but, when he tries to import the second, he
gets an error: "An error occurred while importing".
 I can't find a solution... can anyone help me? Which logs should I
check? Or is it impossible to import 2 or more calendars? 
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