Okay thanks for your advice
I started the upgrade again from 17.1 and have 20.1 version now, working without the access log issues.
I have now the following errors
Call to undefined method EGroupware\Api\Session::appsession().
Please advice how to fix this
Also since phpgwapi has now been removed, I was using the following code in my custom application which now is deprecated. How do I fix this
$this->jscal = createObject('phpgwapi.jscalendar');