Hello Lilian,
I have not touched 4.x till now, but we had a similar problem with the events module in 3.2.5.
It had nothing to do with the database. It was a malfunctioning widget that broke it.
Here is my post from some times ago:
https://openacs.org/bboard/q-and-a-fetch-msg.tcl?msg_id=0000ZS&topic_id=OpenACS&topic=11