Piwik 3.0.1 Error : Failed to get data from the API: SQLSTATE[08S01]

We have a recently upgraded piwik installation on a ubuntu 16 server running php 7.0.13 and mysql 5.717. We are experimenting slowdown and error with the following messages :
Error : Failed to get data from the API: SQLSTATE[08S01]: Communication link failure: 1053 Server shutdown in progress …
Warning : … PDOStatement::Execute(): MySQL server has gone away
Warning : … PDOStatement::Execute(): Error reading result set’s header

Any Idea ?

Best regards,

Didier C