Payplan Upgrade fro 3.7.1 to 4 - 500 error

Hello,

I'm triyng to upgrade Payplan to 4.
I start from Payplan 3.6.2 and Joomla 3.8.10
I've upgraded Payplan to 3.7.1: perfect.
I've upgraded Joomla to 3.9.1: perfect
I've upgraded to Payplan 4: the install procedure went fine, I do not receive errors. All green.
But after upgrade I can't connect to site: error 500.

Any idea?

Thanks,
Mario