Restore full working after a "Mysqli prepare error: INSERT command denied to user" message

Suddenly I received the

Mysqli prepare error: INSERT command denied to user 'xx'@'' for table 'piwik_option'

message. After reading some thread on this topic, I recognized that I run out mysql database space on my host.
Now I have corrected such a problem, having doubled the size dedicated to mysql databases.
All my data web apps work fine, but Piwik refuses to let me logging in, presenting the same message all the times.
How can I restore Piwik operation? If needed, how can I re-do the configuration step, without losing all my collected data?
Thank you.

Ok, maybe it was only a matter of time. Now Piwik comes to working as expected.