Thank you very much, that was so useful!

I checked the trace log, found

!! ERROR: '2006' 'MySQL server has gone away'
[download]

checked what was happening just before it, found...

<- disconnect= 1 at Images.pm line 215
[download]

So, I add

$self->param('session')->close;
[download]

before

$self->param('dbh')->disconnect();
[download]

in my teardown sub, and it's fixed!