Strange behaviour when changing category field

[nalves]

I have a serial chart on my dashboard (portal10.9.1) that behaves unexpectedly when I switch the category from Date1 to Date2.

Date1 and Date2 have the same data type (timestamp) and some of them are equal (10.184.616  from 23,125,500 values and no null values).  When Date1 is selected, the bars are drawn but when I switch to Date2, none of them are created:

Here are my settings for this serial chart:
Categories From: Grouped Values
Category field: Date1
Parse Dates ON
Minimum Period: Month
Statistic: Count
Field: Event_ID

If anyone can help me make sense of that, I would apreciate

Thanks